NOISSUE Flatten gui and logic libraries into MultiMC

launcher/java/JavaChecker.cpp

@@ -0,0 +1,166 @@
+#include "JavaChecker.h"
+#include "JavaUtils.h"
+#include <FileSystem.h>
+#include <Commandline.h>
+#include <QFile>
+#include <QProcess>
+#include <QMap>
+#include <QCoreApplication>
+#include <QDebug>
+
+#include "Env.h"
+
+JavaChecker::JavaChecker(QObject *parent) : QObject(parent)
+{
+}
+
+void JavaChecker::performCheck()
+{
+    QString checkerJar = FS::PathCombine(ENV.getJarsPath(), "JavaCheck.jar");
+
+    QStringList args;
+
+    process.reset(new QProcess());
+    if(m_args.size())
+    {
+        auto extraArgs = Commandline::splitArgs(m_args);
+        args.append(extraArgs);
+    }
+    if(m_minMem != 0)
+    {
+        args << QString("-Xms%1m").arg(m_minMem);
+    }
+    if(m_maxMem != 0)
+    {
+        args << QString("-Xmx%1m").arg(m_maxMem);
+    }
+    if(m_permGen != 64)
+    {
+        args << QString("-XX:PermSize=%1m").arg(m_permGen);
+    }
+
+    args.append({"-jar", checkerJar});
+    process->setArguments(args);
+    process->setProgram(m_path);
+    process->setProcessChannelMode(QProcess::SeparateChannels);
+    process->setProcessEnvironment(CleanEnviroment());
+    qDebug() << "Running java checker: " + m_path + args.join(" ");;
+
+    connect(process.get(), SIGNAL(finished(int, QProcess::ExitStatus)), this, SLOT(finished(int, QProcess::ExitStatus)));
+    connect(process.get(), SIGNAL(error(QProcess::ProcessError)), this, SLOT(error(QProcess::ProcessError)));
+    connect(process.get(), SIGNAL(readyReadStandardOutput()), this, SLOT(stdoutReady()));
+    connect(process.get(), SIGNAL(readyReadStandardError()), this, SLOT(stderrReady()));
+    connect(&killTimer, SIGNAL(timeout()), SLOT(timeout()));
+    killTimer.setSingleShot(true);
+    killTimer.start(15000);
+    process->start();
+}
+
+void JavaChecker::stdoutReady()
+{
+    QByteArray data = process->readAllStandardOutput();
+    QString added = QString::fromLocal8Bit(data);
+    added.remove('\r');
+    m_stdout += added;
+}
+
+void JavaChecker::stderrReady()
+{
+    QByteArray data = process->readAllStandardError();
+    QString added = QString::fromLocal8Bit(data);
+    added.remove('\r');
+    m_stderr += added;
+}
+
+void JavaChecker::finished(int exitcode, QProcess::ExitStatus status)
+{
+    killTimer.stop();
+    QProcessPtr _process = process;
+    process.reset();
+
+    JavaCheckResult result;
+    {
+        result.path = m_path;
+        result.id = m_id;
+    }
+    result.errorLog = m_stderr;
+    result.outLog = m_stdout;
+    qDebug() << "STDOUT" << m_stdout;
+    qWarning() << "STDERR" << m_stderr;
+    qDebug() << "Java checker finished with status " << status << " exit code " << exitcode;
+
+    if (status == QProcess::CrashExit || exitcode == 1)
+    {
+        result.validity = JavaCheckResult::Validity::Errored;
+        emit checkFinished(result);
+        return;
+    }
+
+    bool success = true;
+
+    QMap<QString, QString> results;
+    QStringList lines = m_stdout.split("\n", QString::SkipEmptyParts);
+    for(QString line : lines)
+    {
+        line = line.trimmed();
+
+        auto parts = line.split('=', QString::SkipEmptyParts);
+        if(parts.size() != 2 || parts[0].isEmpty() || parts[1].isEmpty())
+        {
+            success = false;
+        }
+        else
+        {
+            results.insert(parts[0], parts[1]);
+        }
+    }
+
+    if(!results.contains("os.arch") || !results.contains("java.version") || !results.contains("java.vendor") || !success)
+    {
+        result.validity = JavaCheckResult::Validity::ReturnedInvalidData;
+        emit checkFinished(result);
+        return;
+    }
+
+    auto os_arch = results["os.arch"];
+    auto java_version = results["java.version"];
+    auto java_vendor = results["java.vendor"];
+    bool is_64 = os_arch == "x86_64" || os_arch == "amd64";
+
+
+    result.validity = JavaCheckResult::Validity::Valid;
+    result.is_64bit = is_64;
+    result.mojangPlatform = is_64 ? "64" : "32";
+    result.realPlatform = os_arch;
+    result.javaVersion = java_version;
+    result.javaVendor = java_vendor;
+    qDebug() << "Java checker succeeded.";
+    emit checkFinished(result);
+}
+
+void JavaChecker::error(QProcess::ProcessError err)
+{
+    if(err == QProcess::FailedToStart)
+    {
+        killTimer.stop();
+        qDebug() << "Java checker has failed to start.";
+        JavaCheckResult result;
+        {
+            result.path = m_path;
+            result.id = m_id;
+        }
+
+        emit checkFinished(result);
+        return;
+    }
+}
+
+void JavaChecker::timeout()
+{
+    // NO MERCY. NO ABUSE.
+    if(process)
+    {
+        qDebug() << "Java checker has been killed by timeout.";
+        process->kill();
+    }
+}

launcher/java/JavaChecker.h

@@ -0,0 +1,61 @@
+#pragma once
+#include <QProcess>
+#include <QTimer>
+#include <memory>
+
+#include "QObjectPtr.h"
+
+#include "JavaVersion.h"
+
+class JavaChecker;
+
+struct JavaCheckResult
+{
+    QString path;
+    QString mojangPlatform;
+    QString realPlatform;
+    JavaVersion javaVersion;
+    QString javaVendor;
+    QString outLog;
+    QString errorLog;
+    bool is_64bit = false;
+    int id;
+    enum class Validity
+    {
+        Errored,
+        ReturnedInvalidData,
+        Valid
+    } validity = Validity::Errored;
+};
+
+typedef shared_qobject_ptr<QProcess> QProcessPtr;
+typedef shared_qobject_ptr<JavaChecker> JavaCheckerPtr;
+class JavaChecker : public QObject
+{
+    Q_OBJECT
+public:
+    explicit JavaChecker(QObject *parent = 0);
+    void performCheck();
+
+    QString m_path;
+    QString m_args;
+    int m_id = 0;
+    int m_minMem = 0;
+    int m_maxMem = 0;
+    int m_permGen = 64;
+
+signals:
+    void checkFinished(JavaCheckResult result);
+private:
+    QProcessPtr process;
+    QTimer killTimer;
+    QString m_stdout;
+    QString m_stderr;
+public
+slots:
+    void timeout();
+    void finished(int exitcode, QProcess::ExitStatus);
+    void error(QProcess::ProcessError);
+    void stdoutReady();
+    void stderrReady();
+};

launcher/java/JavaCheckerJob.cpp

@@ -0,0 +1,44 @@
+/* Copyright 2013-2021 MultiMC Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "JavaCheckerJob.h"
+
+#include <QDebug>
+
+void JavaCheckerJob::partFinished(JavaCheckResult result)
+{
+    num_finished++;
+    qDebug() << m_job_name.toLocal8Bit() << "progress:" << num_finished << "/"
+                << javacheckers.size();
+    setProgress(num_finished, javacheckers.size());
+
+    javaresults.replace(result.id, result);
+
+    if (num_finished == javacheckers.size())
+    {
+        emitSucceeded();
+    }
+}
+
+void JavaCheckerJob::executeTask()
+{
+    qDebug() << m_job_name.toLocal8Bit() << " started.";
+    for (auto iter : javacheckers)
+    {
+        javaresults.append(JavaCheckResult());
+        connect(iter.get(), SIGNAL(checkFinished(JavaCheckResult)), SLOT(partFinished(JavaCheckResult)));
+        iter->performCheck();
+    }
+}

launcher/java/JavaCheckerJob.h

@@ -0,0 +1,61 @@
+/* Copyright 2013-2021 MultiMC Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <QtNetwork>
+#include "JavaChecker.h"
+#include "tasks/Task.h"
+
+class JavaCheckerJob;
+typedef shared_qobject_ptr<JavaCheckerJob> JavaCheckerJobPtr;
+
+// FIXME: this just seems horribly redundant
+class JavaCheckerJob : public Task
+{
+    Q_OBJECT
+public:
+    explicit JavaCheckerJob(QString job_name) : Task(), m_job_name(job_name) {};
+    virtual ~JavaCheckerJob() {};
+
+    bool addJavaCheckerAction(JavaCheckerPtr base)
+    {
+        javacheckers.append(base);
+        // if this is already running, the action needs to be started right away!
+        if (isRunning())
+        {
+            setProgress(num_finished, javacheckers.size());
+            connect(base.get(), &JavaChecker::checkFinished, this, &JavaCheckerJob::partFinished);
+            base->performCheck();
+        }
+        return true;
+    }
+    QList<JavaCheckResult> getResults()
+    {
+        return javaresults;
+    }
+
+private slots:
+    void partFinished(JavaCheckResult result);
+
+protected:
+    virtual void executeTask() override;
+
+private:
+    QString m_job_name;
+    QList<JavaCheckerPtr> javacheckers;
+    QList<JavaCheckResult> javaresults;
+    int num_finished = 0;
+};

launcher/java/JavaInstall.cpp

@@ -0,0 +1,28 @@
+#include "JavaInstall.h"
+#include <MMCStrings.h>
+
+bool JavaInstall::operator<(const JavaInstall &rhs)
+{
+    auto archCompare = Strings::naturalCompare(arch, rhs.arch, Qt::CaseInsensitive);
+    if(archCompare != 0)
+        return archCompare < 0;
+    if(id < rhs.id)
+    {
+        return true;
+    }
+    if(id > rhs.id)
+    {
+        return false;
+    }
+    return Strings::naturalCompare(path, rhs.path, Qt::CaseInsensitive) < 0;
+}
+
+bool JavaInstall::operator==(const JavaInstall &rhs)
+{
+    return arch == rhs.arch && id == rhs.id && path == rhs.path;
+}
+
+bool JavaInstall::operator>(const JavaInstall &rhs)
+{
+    return (!operator<(rhs)) && (!operator==(rhs));
+}

launcher/java/JavaInstall.h

@@ -0,0 +1,38 @@
+#pragma once
+
+#include "BaseVersion.h"
+#include "JavaVersion.h"
+
+struct JavaInstall : public BaseVersion
+{
+    JavaInstall(){}
+    JavaInstall(QString id, QString arch, QString path)
+    : id(id), arch(arch), path(path)
+    {
+    }
+    virtual QString descriptor()
+    {
+        return id.toString();
+    }
+
+    virtual QString name()
+    {
+        return id.toString();
+    }
+
+    virtual QString typeString() const
+    {
+        return arch;
+    }
+
+    bool operator<(const JavaInstall & rhs);
+    bool operator==(const JavaInstall & rhs);
+    bool operator>(const JavaInstall & rhs);
+
+    JavaVersion id;
+    QString arch;
+    QString path;
+    bool recommended = false;
+};
+
+typedef std::shared_ptr<JavaInstall> JavaInstallPtr;

launcher/java/JavaInstallList.cpp

@@ -0,0 +1,208 @@
+/* Copyright 2013-2021 MultiMC Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <QtNetwork>
+#include <QtXml>
+#include <QRegExp>
+
+#include <QDebug>
+
+#include "java/JavaInstallList.h"
+#include "java/JavaCheckerJob.h"
+#include "java/JavaUtils.h"
+#include "MMCStrings.h"
+#include "minecraft/VersionFilterData.h"
+
+JavaInstallList::JavaInstallList(QObject *parent) : BaseVersionList(parent)
+{
+}
+
+shared_qobject_ptr<Task> JavaInstallList::getLoadTask()
+{
+    load();
+    return getCurrentTask();
+}
+
+shared_qobject_ptr<Task> JavaInstallList::getCurrentTask()
+{
+    if(m_status == Status::InProgress)
+    {
+        return m_loadTask;
+    }
+    return nullptr;
+}
+
+void JavaInstallList::load()
+{
+    if(m_status != Status::InProgress)
+    {
+        m_status = Status::InProgress;
+        m_loadTask = new JavaListLoadTask(this);
+        m_loadTask->start();
+    }
+}
+
+const BaseVersionPtr JavaInstallList::at(int i) const
+{
+    return m_vlist.at(i);
+}
+
+bool JavaInstallList::isLoaded()
+{
+    return m_status == JavaInstallList::Status::Done;
+}
+
+int JavaInstallList::count() const
+{
+    return m_vlist.count();
+}
+
+QVariant JavaInstallList::data(const QModelIndex &index, int role) const
+{
+    if (!index.isValid())
+        return QVariant();
+
+    if (index.row() > count())
+        return QVariant();
+
+    auto version = std::dynamic_pointer_cast<JavaInstall>(m_vlist[index.row()]);
+    switch (role)
+    {
+        case VersionPointerRole:
+            return qVariantFromValue(m_vlist[index.row()]);
+        case VersionIdRole:
+            return version->descriptor();
+        case VersionRole:
+            return version->id.toString();
+        case RecommendedRole:
+            return version->recommended;
+        case PathRole:
+            return version->path;
+        case ArchitectureRole:
+            return version->arch;
+        default:
+            return QVariant();
+    }
+}
+
+BaseVersionList::RoleList JavaInstallList::providesRoles() const
+{
+    return {VersionPointerRole, VersionIdRole, VersionRole, RecommendedRole, PathRole, ArchitectureRole};
+}
+
+
+void JavaInstallList::updateListData(QList<BaseVersionPtr> versions)
+{
+    beginResetModel();
+    m_vlist = versions;
+    sortVersions();
+    if(m_vlist.size())
+    {
+        auto best = std::dynamic_pointer_cast<JavaInstall>(m_vlist[0]);
+        best->recommended = true;
+    }
+    endResetModel();
+    m_status = Status::Done;
+    m_loadTask.reset();
+}
+
+bool sortJavas(BaseVersionPtr left, BaseVersionPtr right)
+{
+    auto rleft = std::dynamic_pointer_cast<JavaInstall>(left);
+    auto rright = std::dynamic_pointer_cast<JavaInstall>(right);
+    return (*rleft) > (*rright);
+}
+
+void JavaInstallList::sortVersions()
+{
+    beginResetModel();
+    std::sort(m_vlist.begin(), m_vlist.end(), sortJavas);
+    endResetModel();
+}
+
+JavaListLoadTask::JavaListLoadTask(JavaInstallList *vlist) : Task()
+{
+    m_list = vlist;
+    m_currentRecommended = NULL;
+}
+
+JavaListLoadTask::~JavaListLoadTask()
+{
+}
+
+void JavaListLoadTask::executeTask()
+{
+    setStatus(tr("Detecting Java installations..."));
+
+    JavaUtils ju;
+    QList<QString> candidate_paths = ju.FindJavaPaths();
+
+    m_job = new JavaCheckerJob("Java detection");
+    connect(m_job.get(), &Task::finished, this, &JavaListLoadTask::javaCheckerFinished);
+    connect(m_job.get(), &Task::progress, this, &Task::setProgress);
+
+    qDebug() << "Probing the following Java paths: ";
+    int id = 0;
+    for(QString candidate : candidate_paths)
+    {
+        qDebug() << " " << candidate;
+
+        auto candidate_checker = new JavaChecker();
+        candidate_checker->m_path = candidate;
+        candidate_checker->m_id = id;
+        m_job->addJavaCheckerAction(JavaCheckerPtr(candidate_checker));
+
+        id++;
+    }
+
+    m_job->start();
+}
+
+void JavaListLoadTask::javaCheckerFinished()
+{
+    QList<JavaInstallPtr> candidates;
+    auto results = m_job->getResults();
+
+    qDebug() << "Found the following valid Java installations:";
+    for(JavaCheckResult result : results)
+    {
+        if(result.validity == JavaCheckResult::Validity::Valid)
+        {
+            JavaInstallPtr javaVersion(new JavaInstall());
+
+            javaVersion->id = result.javaVersion;
+            javaVersion->arch = result.mojangPlatform;
+            javaVersion->path = result.path;
+            candidates.append(javaVersion);
+
+            qDebug() << " " << javaVersion->id.toString() << javaVersion->arch << javaVersion->path;
+        }
+    }
+
+    QList<BaseVersionPtr> javas_bvp;
+    for (auto java : candidates)
+    {
+        //qDebug() << java->id << java->arch << " at " << java->path;
+        BaseVersionPtr bp_java = std::dynamic_pointer_cast<BaseVersion>(java);
+
+        if (bp_java)
+        {
+            javas_bvp.append(java);
+        }
+    }
+
+    m_list->updateListData(javas_bvp);
+    emitSucceeded();
+}

launcher/java/JavaInstallList.h

@@ -0,0 +1,81 @@
+/* Copyright 2013-2021 MultiMC Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <QObject>
+#include <QAbstractListModel>
+
+#include "BaseVersionList.h"
+#include "tasks/Task.h"
+
+#include "JavaCheckerJob.h"
+#include "JavaInstall.h"
+
+#include "QObjectPtr.h"
+
+class JavaListLoadTask;
+
+class JavaInstallList : public BaseVersionList
+{
+    Q_OBJECT
+    enum class Status
+    {
+        NotDone,
+        InProgress,
+        Done
+    };
+public:
+    explicit JavaInstallList(QObject *parent = 0);
+
+    shared_qobject_ptr<Task> getLoadTask() override;
+    bool isLoaded() override;
+    const BaseVersionPtr at(int i) const override;
+    int count() const override;
+    void sortVersions() override;
+
+    QVariant data(const QModelIndex &index, int role) const override;
+    RoleList providesRoles() const override;
+
+public slots:
+    void updateListData(QList<BaseVersionPtr> versions) override;
+
+protected:
+    void load();
+    shared_qobject_ptr<Task> getCurrentTask();
+
+protected:
+    Status m_status = Status::NotDone;
+    shared_qobject_ptr<JavaListLoadTask> m_loadTask;
+    QList<BaseVersionPtr> m_vlist;
+};
+
+class JavaListLoadTask : public Task
+{
+    Q_OBJECT
+
+public:
+    explicit JavaListLoadTask(JavaInstallList *vlist);
+    virtual ~JavaListLoadTask();
+
+    void executeTask() override;
+public slots:
+    void javaCheckerFinished();
+
+protected:
+    shared_qobject_ptr<JavaCheckerJob> m_job;
+    JavaInstallList *m_list;
+    JavaInstall *m_currentRecommended;
+};

launcher/java/JavaUtils.cpp

@@ -0,0 +1,399 @@
+/* Copyright 2013-2021 MultiMC Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <QStringList>
+#include <QString>
+#include <QDir>
+#include <QStringList>
+
+#include <settings/Setting.h>
+
+#include <QDebug>
+#include "java/JavaUtils.h"
+#include "java/JavaInstallList.h"
+#include "FileSystem.h"
+
+#define IBUS "@im=ibus"
+
+JavaUtils::JavaUtils()
+{
+}
+
+#ifdef Q_OS_LINUX
+static QString processLD_LIBRARY_PATH(const QString & LD_LIBRARY_PATH)
+{
+    QDir mmcBin(QCoreApplication::applicationDirPath());
+    auto items = LD_LIBRARY_PATH.split(':');
+    QStringList final;
+    for(auto & item: items)
+    {
+        QDir test(item);
+        if(test == mmcBin)
+        {
+            qDebug() << "Env:LD_LIBRARY_PATH ignoring path" << item;
+            continue;
+        }
+        final.append(item);
+    }
+    return final.join(':');
+}
+#endif
+
+QProcessEnvironment CleanEnviroment()
+{
+    // prepare the process environment
+    QProcessEnvironment rawenv = QProcessEnvironment::systemEnvironment();
+    QProcessEnvironment env;
+
+    QStringList ignored =
+    {
+        "JAVA_ARGS",
+        "CLASSPATH",
+        "CONFIGPATH",
+        "JAVA_HOME",
+        "JRE_HOME",
+        "_JAVA_OPTIONS",
+        "JAVA_OPTIONS",
+        "JAVA_TOOL_OPTIONS"
+    };
+    for(auto key: rawenv.keys())
+    {
+        auto value = rawenv.value(key);
+        // filter out dangerous java crap
+        if(ignored.contains(key))
+        {
+            qDebug() << "Env: ignoring" << key << value;
+            continue;
+        }
+        // filter MultiMC-related things
+        if(key.startsWith("QT_"))
+        {
+            qDebug() << "Env: ignoring" << key << value;
+            continue;
+        }
+#ifdef Q_OS_LINUX
+        // Do not pass LD_* variables to java. They were intended for MultiMC
+        if(key.startsWith("LD_"))
+        {
+            qDebug() << "Env: ignoring" << key << value;
+            continue;
+        }
+        // Strip IBus
+        // IBus is a Linux IME framework. For some reason, it breaks MC?
+        if (key == "XMODIFIERS" && value.contains(IBUS))
+        {
+            QString save = value;
+            value.replace(IBUS, "");
+            qDebug() << "Env: stripped" << IBUS << "from" << save << ":" << value;
+        }
+        if(key == "GAME_PRELOAD")
+        {
+            env.insert("LD_PRELOAD", value);
+            continue;
+        }
+        if(key == "GAME_LIBRARY_PATH")
+        {
+            env.insert("LD_LIBRARY_PATH", processLD_LIBRARY_PATH(value));
+            continue;
+        }
+#endif
+        // qDebug() << "Env: " << key << value;
+        env.insert(key, value);
+    }
+#ifdef Q_OS_LINUX
+    // HACK: Workaround for QTBUG42500
+    if(!env.contains("LD_LIBRARY_PATH"))
+    {
+        env.insert("LD_LIBRARY_PATH", "");
+    }
+#endif
+
+    return env;
+}
+
+JavaInstallPtr JavaUtils::MakeJavaPtr(QString path, QString id, QString arch)
+{
+    JavaInstallPtr javaVersion(new JavaInstall());
+
+    javaVersion->id = id;
+    javaVersion->arch = arch;
+    javaVersion->path = path;
+
+    return javaVersion;
+}
+
+JavaInstallPtr JavaUtils::GetDefaultJava()
+{
+    JavaInstallPtr javaVersion(new JavaInstall());
+
+    javaVersion->id = "java";
+    javaVersion->arch = "unknown";
+#if defined(Q_OS_WIN32)
+    javaVersion->path = "javaw";
+#else
+    javaVersion->path = "java";
+#endif
+
+    return javaVersion;
+}
+
+#if defined(Q_OS_WIN32)
+QList<JavaInstallPtr> JavaUtils::FindJavaFromRegistryKey(DWORD keyType, QString keyName, QString keyJavaDir, QString subkeySuffix)
+{
+    QList<JavaInstallPtr> javas;
+
+    QString archType = "unknown";
+    if (keyType == KEY_WOW64_64KEY)
+        archType = "64";
+    else if (keyType == KEY_WOW64_32KEY)
+        archType = "32";
+
+    HKEY jreKey;
+    if (RegOpenKeyExA(HKEY_LOCAL_MACHINE, keyName.toStdString().c_str(), 0,
+                      KEY_READ | keyType | KEY_ENUMERATE_SUB_KEYS, &jreKey) == ERROR_SUCCESS)
+    {
+        // Read the current type version from the registry.
+        // This will be used to find any key that contains the JavaHome value.
+        char *value = new char[0];
+        DWORD valueSz = 0;
+        if (RegQueryValueExA(jreKey, "CurrentVersion", NULL, NULL, (BYTE *)value, &valueSz) ==
+            ERROR_MORE_DATA)
+        {
+            value = new char[valueSz];
+            RegQueryValueExA(jreKey, "CurrentVersion", NULL, NULL, (BYTE *)value, &valueSz);
+        }
+
+        TCHAR subKeyName[255];
+        DWORD subKeyNameSize, numSubKeys, retCode;
+
+        // Get the number of subkeys
+        RegQueryInfoKey(jreKey, NULL, NULL, NULL, &numSubKeys, NULL, NULL, NULL, NULL, NULL,
+                        NULL, NULL);
+
+        // Iterate until RegEnumKeyEx fails
+        if (numSubKeys > 0)
+        {
+            for (DWORD i = 0; i < numSubKeys; i++)
+            {
+                subKeyNameSize = 255;
+                retCode = RegEnumKeyEx(jreKey, i, subKeyName, &subKeyNameSize, NULL, NULL, NULL,
+                                       NULL);
+                if (retCode == ERROR_SUCCESS)
+                {
+                    // Now open the registry key for the version that we just got.
+                    QString newKeyName = keyName + "\\" + subKeyName + subkeySuffix;
+
+                    HKEY newKey;
+                    if (RegOpenKeyEx(HKEY_LOCAL_MACHINE, newKeyName.toStdString().c_str(), 0,
+                                     KEY_READ | KEY_WOW64_64KEY, &newKey) == ERROR_SUCCESS)
+                    {
+                        // Read the JavaHome value to find where Java is installed.
+                        value = new char[0];
+                        valueSz = 0;
+                        if (RegQueryValueEx(newKey, keyJavaDir.toStdString().c_str(), NULL, NULL, (BYTE *)value,
+                                            &valueSz) == ERROR_MORE_DATA)
+                        {
+                            value = new char[valueSz];
+                            RegQueryValueEx(newKey, keyJavaDir.toStdString().c_str(), NULL, NULL, (BYTE *)value,
+                                            &valueSz);
+
+                            // Now, we construct the version object and add it to the list.
+                            JavaInstallPtr javaVersion(new JavaInstall());
+
+                            javaVersion->id = subKeyName;
+                            javaVersion->arch = archType;
+                            javaVersion->path =
+                                QDir(FS::PathCombine(value, "bin")).absoluteFilePath("javaw.exe");
+                            javas.append(javaVersion);
+                        }
+
+                        RegCloseKey(newKey);
+                    }
+                }
+            }
+        }
+
+        RegCloseKey(jreKey);
+    }
+
+    return javas;
+}
+
+QList<QString> JavaUtils::FindJavaPaths()
+{
+    QList<JavaInstallPtr> java_candidates;
+
+    // Oracle
+    QList<JavaInstallPtr> JRE64s = this->FindJavaFromRegistryKey(
+        KEY_WOW64_64KEY, "SOFTWARE\\JavaSoft\\Java Runtime Environment", "JavaHome");
+    QList<JavaInstallPtr> JDK64s = this->FindJavaFromRegistryKey(
+        KEY_WOW64_64KEY, "SOFTWARE\\JavaSoft\\Java Development Kit", "JavaHome");
+    QList<JavaInstallPtr> JRE32s = this->FindJavaFromRegistryKey(
+        KEY_WOW64_32KEY, "SOFTWARE\\JavaSoft\\Java Runtime Environment", "JavaHome");
+    QList<JavaInstallPtr> JDK32s = this->FindJavaFromRegistryKey(
+        KEY_WOW64_32KEY, "SOFTWARE\\JavaSoft\\Java Development Kit", "JavaHome");
+
+    // Oracle for Java 9 and newer
+    QList<JavaInstallPtr> NEWJRE64s = this->FindJavaFromRegistryKey(
+        KEY_WOW64_64KEY, "SOFTWARE\\JavaSoft\\JRE", "JavaHome");
+    QList<JavaInstallPtr> NEWJDK64s = this->FindJavaFromRegistryKey(
+        KEY_WOW64_64KEY, "SOFTWARE\\JavaSoft\\JDK", "JavaHome");
+    QList<JavaInstallPtr> NEWJRE32s = this->FindJavaFromRegistryKey(
+        KEY_WOW64_32KEY, "SOFTWARE\\JavaSoft\\JRE", "JavaHome");
+    QList<JavaInstallPtr> NEWJDK32s = this->FindJavaFromRegistryKey(
+        KEY_WOW64_32KEY, "SOFTWARE\\JavaSoft\\JDK", "JavaHome");
+
+    // AdoptOpenJDK
+    QList<JavaInstallPtr> ADOPTOPENJRE32s = this->FindJavaFromRegistryKey(
+        KEY_WOW64_32KEY, "SOFTWARE\\AdoptOpenJDK\\JRE", "Path", "\\hotspot\\MSI");
+    QList<JavaInstallPtr> ADOPTOPENJRE64s = this->FindJavaFromRegistryKey(
+        KEY_WOW64_64KEY, "SOFTWARE\\AdoptOpenJDK\\JRE", "Path", "\\hotspot\\MSI");
+    QList<JavaInstallPtr> ADOPTOPENJDK32s = this->FindJavaFromRegistryKey(
+        KEY_WOW64_32KEY, "SOFTWARE\\AdoptOpenJDK\\JDK", "Path", "\\hotspot\\MSI");
+    QList<JavaInstallPtr> ADOPTOPENJDK64s = this->FindJavaFromRegistryKey(
+        KEY_WOW64_64KEY, "SOFTWARE\\AdoptOpenJDK\\JDK", "Path", "\\hotspot\\MSI");
+
+    // Microsoft
+    QList<JavaInstallPtr> MICROSOFTJDK64s = this->FindJavaFromRegistryKey(
+        KEY_WOW64_64KEY, "SOFTWARE\\Microsoft\\JDK", "Path", "\\hotspot\\MSI");
+
+    // Azul Zulu
+    QList<JavaInstallPtr> ZULU64s = this->FindJavaFromRegistryKey(
+        KEY_WOW64_64KEY, "SOFTWARE\\Azul Systems\\Zulu", "InstallationPath");
+    QList<JavaInstallPtr> ZULU32s = this->FindJavaFromRegistryKey(
+        KEY_WOW64_32KEY, "SOFTWARE\\Azul Systems\\Zulu", "InstallationPath");
+    
+    // BellSoft Liberica
+    QList<JavaInstallPtr> LIBERICA64s = this->FindJavaFromRegistryKey(
+        KEY_WOW64_64KEY, "SOFTWARE\\BellSoft\\Liberica", "InstallationPath");
+    QList<JavaInstallPtr> LIBERICA32s = this->FindJavaFromRegistryKey(
+        KEY_WOW64_32KEY, "SOFTWARE\\BellSoft\\Liberica", "InstallationPath");
+
+    // List x64 before x86
+    java_candidates.append(JRE64s);
+    java_candidates.append(NEWJRE64s);
+    java_candidates.append(ADOPTOPENJRE64s);
+    java_candidates.append(MakeJavaPtr("C:/Program Files/Java/jre8/bin/javaw.exe"));
+    java_candidates.append(MakeJavaPtr("C:/Program Files/Java/jre7/bin/javaw.exe"));
+    java_candidates.append(MakeJavaPtr("C:/Program Files/Java/jre6/bin/javaw.exe"));
+    java_candidates.append(JDK64s);
+    java_candidates.append(NEWJDK64s);
+    java_candidates.append(ADOPTOPENJDK64s);
+    java_candidates.append(MICROSOFTJDK64s);
+    java_candidates.append(ZULU64s);
+    java_candidates.append(LIBERICA64s);
+
+    java_candidates.append(JRE32s);
+    java_candidates.append(NEWJRE32s);
+    java_candidates.append(ADOPTOPENJRE32s);
+    java_candidates.append(MakeJavaPtr("C:/Program Files (x86)/Java/jre8/bin/javaw.exe"));
+    java_candidates.append(MakeJavaPtr("C:/Program Files (x86)/Java/jre7/bin/javaw.exe"));
+    java_candidates.append(MakeJavaPtr("C:/Program Files (x86)/Java/jre6/bin/javaw.exe"));
+    java_candidates.append(JDK32s);
+    java_candidates.append(NEWJDK32s);
+    java_candidates.append(ADOPTOPENJDK32s);
+    java_candidates.append(ZULU32s);
+    java_candidates.append(LIBERICA32s);
+    
+    java_candidates.append(MakeJavaPtr(this->GetDefaultJava()->path));
+
+    QList<QString> candidates;
+    for(JavaInstallPtr java_candidate : java_candidates)
+    {
+        if(!candidates.contains(java_candidate->path))
+        {
+            candidates.append(java_candidate->path);
+        }
+    }
+
+    return candidates;
+}
+
+#elif defined(Q_OS_MAC)
+QList<QString> JavaUtils::FindJavaPaths()
+{
+    QList<QString> javas;
+    javas.append(this->GetDefaultJava()->path);
+    javas.append("/Applications/Xcode.app/Contents/Applications/Application Loader.app/Contents/MacOS/itms/java/bin/java");
+    javas.append("/Library/Internet Plug-Ins/JavaAppletPlugin.plugin/Contents/Home/bin/java");
+    javas.append("/System/Library/Frameworks/JavaVM.framework/Versions/Current/Commands/java");
+    QDir libraryJVMDir("/Library/Java/JavaVirtualMachines/");
+    QStringList libraryJVMJavas = libraryJVMDir.entryList(QDir::Dirs | QDir::NoDotAndDotDot);
+    foreach (const QString &java, libraryJVMJavas) {
+        javas.append(libraryJVMDir.absolutePath() + "/" + java + "/Contents/Home/bin/java");
+        javas.append(libraryJVMDir.absolutePath() + "/" + java + "/Contents/Home/jre/bin/java");
+    }
+    QDir systemLibraryJVMDir("/System/Library/Java/JavaVirtualMachines/");
+    QStringList systemLibraryJVMJavas = systemLibraryJVMDir.entryList(QDir::Dirs | QDir::NoDotAndDotDot);
+    foreach (const QString &java, systemLibraryJVMJavas) {
+        javas.append(systemLibraryJVMDir.absolutePath() + "/" + java + "/Contents/Home/bin/java");
+        javas.append(systemLibraryJVMDir.absolutePath() + "/" + java + "/Contents/Commands/java");
+    }
+    return javas;
+}
+
+#elif defined(Q_OS_LINUX)
+QList<QString> JavaUtils::FindJavaPaths()
+{
+    qDebug() << "Linux Java detection incomplete - defaulting to \"java\"";
+
+    QList<QString> javas;
+    javas.append(this->GetDefaultJava()->path);
+    auto scanJavaDir = [&](const QString & dirPath)
+    {
+        QDir dir(dirPath);
+        if(!dir.exists())
+            return;
+        auto entries = dir.entryInfoList(QDir::Dirs | QDir::NoDotAndDotDot | QDir::NoSymLinks);
+        for(auto & entry: entries)
+        {
+
+            QString prefix;
+            if(entry.isAbsolute())
+            {
+                prefix = entry.absoluteFilePath();
+            }
+            else
+            {
+                prefix = entry.filePath();
+            }
+
+            javas.append(FS::PathCombine(prefix, "jre/bin/java"));
+            javas.append(FS::PathCombine(prefix, "bin/java"));
+        }
+    };
+    // oracle RPMs
+    scanJavaDir("/usr/java");
+    // general locations used by distro packaging
+    scanJavaDir("/usr/lib/jvm");
+    scanJavaDir("/usr/lib32/jvm");
+    // javas stored in MultiMC's folder
+    scanJavaDir("java");
+    // manually installed JDKs in /opt
+    scanJavaDir("/opt/jdk");
+    scanJavaDir("/opt/jdks");
+    return javas;
+}
+#else
+QList<QString> JavaUtils::FindJavaPaths()
+{
+    qDebug() << "Unknown operating system build - defaulting to \"java\"";
+
+    QList<QString> javas;
+    javas.append(this->GetDefaultJava()->path);
+
+    return javas;
+}
+#endif

launcher/java/JavaUtils.h

@@ -0,0 +1,42 @@
+/* Copyright 2013-2021 MultiMC Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <QStringList>
+
+#include "JavaChecker.h"
+#include "JavaInstallList.h"
+
+#ifdef Q_OS_WIN
+#include <windows.h>
+#endif
+
+QProcessEnvironment CleanEnviroment();
+
+class JavaUtils : public QObject
+{
+    Q_OBJECT
+public:
+    JavaUtils();
+
+    JavaInstallPtr MakeJavaPtr(QString path, QString id = "unknown", QString arch = "unknown");
+    QList<QString> FindJavaPaths();
+    JavaInstallPtr GetDefaultJava();
+
+#ifdef Q_OS_WIN
+    QList<JavaInstallPtr> FindJavaFromRegistryKey(DWORD keyType, QString keyName, QString keyJavaDir, QString subkeySuffix = "");
+#endif
+};

launcher/java/JavaVersion.cpp

@@ -0,0 +1,121 @@
+#include "JavaVersion.h"
+#include <MMCStrings.h>
+
+#include <QRegularExpression>
+#include <QString>
+
+JavaVersion & JavaVersion::operator=(const QString & javaVersionString)
+{
+    m_string = javaVersionString;
+
+    auto getCapturedInteger = [](const QRegularExpressionMatch & match, const QString &what) -> int
+    {
+        auto str = match.captured(what);
+        if(str.isEmpty())
+        {
+            return 0;
+        }
+        return str.toInt();
+    };
+
+    QRegularExpression pattern;
+    if(javaVersionString.startsWith("1."))
+    {
+        pattern = QRegularExpression ("1[.](?<major>[0-9]+)([.](?<minor>[0-9]+))?(_(?<security>[0-9]+)?)?(-(?<prerelease>[a-zA-Z0-9]+))?");
+    }
+    else
+    {
+        pattern = QRegularExpression("(?<major>[0-9]+)([.](?<minor>[0-9]+))?([.](?<security>[0-9]+))?(-(?<prerelease>[a-zA-Z0-9]+))?");
+    }
+
+    auto match = pattern.match(m_string);
+    m_parseable = match.hasMatch();
+    m_major = getCapturedInteger(match, "major");
+    m_minor = getCapturedInteger(match, "minor");
+    m_security = getCapturedInteger(match, "security");
+    m_prerelease = match.captured("prerelease");
+    return *this;
+}
+
+JavaVersion::JavaVersion(const QString &rhs)
+{
+    operator=(rhs);
+}
+
+QString JavaVersion::toString()
+{
+    return m_string;
+}
+
+bool JavaVersion::requiresPermGen()
+{
+    if(m_parseable)
+    {
+        return m_major < 8;
+    }
+    return true;
+}
+
+bool JavaVersion::operator<(const JavaVersion &rhs)
+{
+    if(m_parseable && rhs.m_parseable)
+    {
+        auto major = m_major;
+        auto rmajor = rhs.m_major;
+
+        // HACK: discourage using java 9
+        if(major > 8)
+            major = -major;
+        if(rmajor > 8)
+            rmajor = -rmajor;
+
+        if(major < rmajor)
+            return true;
+        if(major > rmajor)
+            return false;
+        if(m_minor < rhs.m_minor)
+            return true;
+        if(m_minor > rhs.m_minor)
+            return false;
+        if(m_security < rhs.m_security)
+            return true;
+        if(m_security > rhs.m_security)
+            return false;
+
+        // everything else being equal, consider prerelease status
+        bool thisPre = !m_prerelease.isEmpty();
+        bool rhsPre = !rhs.m_prerelease.isEmpty();
+        if(thisPre && !rhsPre)
+        {
+            // this is a prerelease and the other one isn't -> lesser
+            return true;
+        }
+        else if(!thisPre && rhsPre)
+        {
+            // this isn't a prerelease and the other one is -> greater
+            return false;
+        }
+        else if(thisPre && rhsPre)
+        {
+            // both are prereleases - use natural compare...
+            return Strings::naturalCompare(m_prerelease, rhs.m_prerelease, Qt::CaseSensitive) < 0;
+        }
+        // neither is prerelease, so they are the same -> this cannot be less than rhs
+        return false;
+    }
+    else return Strings::naturalCompare(m_string, rhs.m_string, Qt::CaseSensitive) < 0;
+}
+
+bool JavaVersion::operator==(const JavaVersion &rhs)
+{
+    if(m_parseable && rhs.m_parseable)
+    {
+        return m_major == rhs.m_major && m_minor == rhs.m_minor && m_security == rhs.m_security && m_prerelease == rhs.m_prerelease;
+    }
+    return m_string == rhs.m_string;
+}
+
+bool JavaVersion::operator>(const JavaVersion &rhs)
+{
+    return (!operator<(rhs)) && (!operator==(rhs));
+}

launcher/java/JavaVersion.h

@@ -0,0 +1,49 @@
+#pragma once
+
+#include <QString>
+
+// NOTE: apparently the GNU C library pollutes the global namespace with these... undef them.
+#ifdef major
+    #undef major
+#endif
+#ifdef minor
+    #undef minor
+#endif
+
+class JavaVersion
+{
+    friend class JavaVersionTest;
+public:
+    JavaVersion() {};
+    JavaVersion(const QString & rhs);
+
+    JavaVersion & operator=(const QString & rhs);
+
+    bool operator<(const JavaVersion & rhs);
+    bool operator==(const JavaVersion & rhs);
+    bool operator>(const JavaVersion & rhs);
+
+    bool requiresPermGen();
+
+    QString toString();
+
+    int major()
+    {
+        return m_major;
+    }
+    int minor()
+    {
+        return m_minor;
+    }
+    int security()
+    {
+        return m_security;
+    }
+private:
+    QString m_string;
+    int m_major = 0;
+    int m_minor = 0;
+    int m_security = 0;
+    bool m_parseable = false;
+    QString m_prerelease;
+};

launcher/java/JavaVersion_test.cpp

@@ -0,0 +1,116 @@
+#include <QTest>
+#include "TestUtil.h"
+
+#include "java/JavaVersion.h"
+
+class JavaVersionTest : public QObject
+{
+    Q_OBJECT
+private
+slots:
+    void test_Parse_data()
+    {
+        QTest::addColumn<QString>("string");
+        QTest::addColumn<int>("major");
+        QTest::addColumn<int>("minor");
+        QTest::addColumn<int>("security");
+        QTest::addColumn<QString>("prerelease");
+
+        QTest::newRow("old format") << "1.6.0_33" << 6 << 0 << 33 << QString();
+        QTest::newRow("old format prerelease") << "1.9.0_1-ea" << 9 << 0 << 1 << "ea";
+
+        QTest::newRow("new format major") << "9" << 9 << 0 << 0 << QString();
+        QTest::newRow("new format minor") << "9.1" << 9 << 1 << 0 << QString();
+        QTest::newRow("new format security") << "9.0.1" << 9 << 0 << 1 << QString();
+        QTest::newRow("new format prerelease") << "9-ea" << 9 << 0 << 0 << "ea";
+        QTest::newRow("new format long prerelease") << "9.0.1-ea" << 9 << 0 << 1 << "ea";
+    }
+    void test_Parse()
+    {
+        QFETCH(QString, string);
+        QFETCH(int, major);
+        QFETCH(int, minor);
+        QFETCH(int, security);
+        QFETCH(QString, prerelease);
+
+        JavaVersion test(string);
+        QCOMPARE(test.m_string, string);
+        QCOMPARE(test.toString(), string);
+        QCOMPARE(test.m_major, major);
+        QCOMPARE(test.m_minor, minor);
+        QCOMPARE(test.m_security, security);
+        QCOMPARE(test.m_prerelease, prerelease);
+    }
+
+    void test_Sort_data()
+    {
+        QTest::addColumn<QString>("lhs");
+        QTest::addColumn<QString>("rhs");
+        QTest::addColumn<bool>("smaller");
+        QTest::addColumn<bool>("equal");
+        QTest::addColumn<bool>("bigger");
+
+        // old format and new format equivalence
+        QTest::newRow("1.6.0_33 == 6.0.33") << "1.6.0_33" << "6.0.33" << false << true << false;
+        // old format major version
+        QTest::newRow("1.5.0_33 < 1.6.0_33") << "1.5.0_33" << "1.6.0_33" << true << false << false;
+        // new format - first release vs first security patch
+        QTest::newRow("9 < 9.0.1") << "9" << "9.0.1" << true << false << false;
+        QTest::newRow("9.0.1 > 9") << "9.0.1" << "9" << false << false << true;
+        // new format - first minor vs first release/security patch
+        QTest::newRow("9.1 > 9.0.1") << "9.1" << "9.0.1" << false << false << true;
+        QTest::newRow("9.0.1 < 9.1") << "9.0.1" << "9.1" << true << false << false;
+        QTest::newRow("9.1 > 9") << "9.1" << "9" << false << false << true;
+        QTest::newRow("9 > 9.1") << "9" << "9.1" << true << false << false;
+        // new format - omitted numbers
+        QTest::newRow("9 == 9.0") << "9" << "9.0" << false << true << false;
+        QTest::newRow("9 == 9.0.0") << "9" << "9.0.0" << false << true << false;
+        QTest::newRow("9.0 == 9.0.0") << "9.0" << "9.0.0" << false << true << false;
+        // early access and prereleases compared to final release
+        QTest::newRow("9-ea < 9") << "9-ea" << "9" << true << false << false;
+        QTest::newRow("9 < 9.0.1-ea") << "9" << "9.0.1-ea" << true << false << false;
+        QTest::newRow("9.0.1-ea > 9") << "9.0.1-ea" << "9" << false << false << true;
+        // prerelease difference only testing
+        QTest::newRow("9-1 == 9-1") << "9-1" << "9-1" << false << true << false;
+        QTest::newRow("9-1 < 9-2") << "9-1" << "9-2" << true << false << false;
+        QTest::newRow("9-5 < 9-20") << "9-5" << "9-20" << true << false << false;
+        QTest::newRow("9-rc1 < 9-rc2") << "9-rc1" << "9-rc2" << true << false << false;
+        QTest::newRow("9-rc5 < 9-rc20") << "9-rc5" << "9-rc20" << true << false << false;
+        QTest::newRow("9-rc < 9-rc2") << "9-rc" << "9-rc2" << true << false << false;
+        QTest::newRow("9-ea < 9-rc") << "9-ea" << "9-rc" << true << false << false;
+    }
+    void test_Sort()
+    {
+        QFETCH(QString, lhs);
+        QFETCH(QString, rhs);
+        QFETCH(bool, smaller);
+        QFETCH(bool, equal);
+        QFETCH(bool, bigger);
+        JavaVersion lver(lhs);
+        JavaVersion rver(rhs);
+        QCOMPARE(lver < rver, smaller);
+        QCOMPARE(lver == rver, equal);
+        QCOMPARE(lver > rver, bigger);
+    }
+    void test_PermGen_data()
+    {
+        QTest::addColumn<QString>("version");
+        QTest::addColumn<bool>("needs_permgen");
+        QTest::newRow("1.6.0_33") << "1.6.0_33" << true;
+        QTest::newRow("1.7.0_60") << "1.7.0_60" << true;
+        QTest::newRow("1.8.0_22") << "1.8.0_22" << false;
+        QTest::newRow("9-ea") << "9-ea" << false;
+        QTest::newRow("9.2.4") << "9.2.4" << false;
+    }
+    void test_PermGen()
+    {
+        QFETCH(QString, version);
+        QFETCH(bool, needs_permgen);
+        JavaVersion v(version);
+        QCOMPARE(needs_permgen, v.requiresPermGen());
+    }
+};
+
+QTEST_GUILESS_MAIN(JavaVersionTest)
+
+#include "JavaVersion_test.moc"

launcher/java/launch/CheckJava.cpp

@@ -0,0 +1,139 @@
+/* Copyright 2013-2021 MultiMC Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "CheckJava.h"
+#include <launch/LaunchTask.h>
+#include <FileSystem.h>
+#include <QStandardPaths>
+#include <QFileInfo>
+#include <sys.h>
+
+void CheckJava::executeTask()
+{
+    auto instance = m_parent->instance();
+    auto settings = instance->settings();
+    m_javaPath = FS::ResolveExecutable(settings->get("JavaPath").toString());
+    bool perInstance = settings->get("OverrideJava").toBool() || settings->get("OverrideJavaLocation").toBool();
+
+    auto realJavaPath = QStandardPaths::findExecutable(m_javaPath);
+    if (realJavaPath.isEmpty())
+    {
+        if (perInstance)
+        {
+            emit logLine(
+                QString("The java binary \"%1\" couldn't be found. Please fix the java path "
+                   "override in the instance's settings or disable it.").arg(m_javaPath),
+                MessageLevel::Warning);
+        }
+        else
+        {
+            emit logLine(QString("The java binary \"%1\" couldn't be found. Please set up java in "
+                            "the settings.").arg(m_javaPath),
+                         MessageLevel::Warning);
+        }
+        emitFailed(QString("Java path is not valid."));
+        return;
+    }
+    else
+    {
+        emit logLine("Java path is:\n" + m_javaPath + "\n\n", MessageLevel::MultiMC);
+    }
+
+    QFileInfo javaInfo(realJavaPath);
+    qlonglong javaUnixTime = javaInfo.lastModified().toMSecsSinceEpoch();
+    auto storedUnixTime = settings->get("JavaTimestamp").toLongLong();
+    auto storedArchitecture = settings->get("JavaArchitecture").toString();
+    auto storedVersion = settings->get("JavaVersion").toString();
+    auto storedVendor = settings->get("JavaVendor").toString();
+    m_javaUnixTime = javaUnixTime;
+    // if timestamps are not the same, or something is missing, check!
+    if (javaUnixTime != storedUnixTime || storedVersion.size() == 0 || storedArchitecture.size() == 0 || storedVendor.size() == 0)
+    {
+        m_JavaChecker = new JavaChecker();
+        emit logLine(QString("Checking Java version..."), MessageLevel::MultiMC);
+        connect(m_JavaChecker.get(), &JavaChecker::checkFinished, this, &CheckJava::checkJavaFinished);
+        m_JavaChecker->m_path = realJavaPath;
+        m_JavaChecker->performCheck();
+        return;
+    }
+    else
+    {
+        auto verString = instance->settings()->get("JavaVersion").toString();
+        auto archString = instance->settings()->get("JavaArchitecture").toString();
+        auto vendorString = instance->settings()->get("JavaVendor").toString();
+        printJavaInfo(verString, archString, vendorString);
+    }
+    emitSucceeded();
+}
+
+void CheckJava::checkJavaFinished(JavaCheckResult result)
+{
+    switch (result.validity)
+    {
+        case JavaCheckResult::Validity::Errored:
+        {
+            // Error message displayed if java can't start
+            emit logLine(QString("Could not start java:"), MessageLevel::Error);
+            emit logLines(result.errorLog.split('\n'), MessageLevel::Error);
+            emit logLine("\nCheck your MultiMC Java settings.", MessageLevel::MultiMC);
+            printSystemInfo(false, false);
+            emitFailed(QString("Could not start java!"));
+            return;
+        }
+        case JavaCheckResult::Validity::ReturnedInvalidData:
+        {
+            emit logLine(QString("Java checker returned some invalid data MultiMC doesn't understand:"), MessageLevel::Error);
+            emit logLines(result.outLog.split('\n'), MessageLevel::Warning);
+            emit logLine("\nMinecraft might not start properly.", MessageLevel::MultiMC);
+            printSystemInfo(false, false);
+            emitSucceeded();
+            return;
+        }
+        case JavaCheckResult::Validity::Valid:
+        {
+            auto instance = m_parent->instance();
+            printJavaInfo(result.javaVersion.toString(), result.mojangPlatform, result.javaVendor);
+            instance->settings()->set("JavaVersion", result.javaVersion.toString());
+            instance->settings()->set("JavaArchitecture", result.mojangPlatform);
+            instance->settings()->set("JavaVendor", result.javaVendor);
+            instance->settings()->set("JavaTimestamp", m_javaUnixTime);
+            emitSucceeded();
+            return;
+        }
+    }
+}
+
+void CheckJava::printJavaInfo(const QString& version, const QString& architecture, const QString & vendor)
+{
+    emit logLine(QString("Java is version %1, using %2-bit architecture, from %3.\n\n").arg(version, architecture, vendor), MessageLevel::MultiMC);
+    printSystemInfo(true, architecture == "64");
+}
+
+void CheckJava::printSystemInfo(bool javaIsKnown, bool javaIs64bit)
+{
+    auto cpu64 = Sys::isCPU64bit();
+    auto system64 = Sys::isSystem64bit();
+    if(cpu64 != system64)
+    {
+        emit logLine(QString("Your CPU architecture is not matching your system architecture. You might want to install a 64bit Operating System.\n\n"), MessageLevel::Error);
+    }
+    if(javaIsKnown)
+    {
+        if(javaIs64bit != system64)
+        {
+            emit logLine(QString("Your Java architecture is not matching your system architecture. You might want to install a 64bit Java version.\n\n"), MessageLevel::Error);
+        }
+    }
+}

launcher/java/launch/CheckJava.h

@@ -0,0 +1,45 @@
+/* Copyright 2013-2021 MultiMC Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <launch/LaunchStep.h>
+#include <LoggedProcess.h>
+#include <java/JavaChecker.h>
+
+class CheckJava: public LaunchStep
+{
+    Q_OBJECT
+public:
+    explicit CheckJava(LaunchTask *parent) :LaunchStep(parent){};
+    virtual ~CheckJava() {};
+
+    virtual void executeTask();
+    virtual bool canAbort() const
+    {
+        return false;
+    }
+private slots:
+    void checkJavaFinished(JavaCheckResult result);
+
+private:
+    void printJavaInfo(const QString & version, const QString & architecture, const QString & vendor);
+    void printSystemInfo(bool javaIsKnown, bool javaIs64bit);
+
+private:
+    QString m_javaPath;
+    qlonglong m_javaUnixTime;
+    JavaCheckerPtr m_JavaChecker;
+};