JVM系列(二)：jvm是如何加載java代碼的？

// share/bin/java.c// 加載 main 函數(shù)類// 通過引入 JavaMain(), 接入java方法// #define JNICALL __stdcallint JNICALLJavaMain(void * _args){    JavaMainArgs *args = (JavaMainArgs *)_args;    int argc = args->argc;    char **argv = args->argv;    int mode = args->mode;    char *what = args->what;    // 一些jvm的調(diào)用實(shí)例，在之前的步驟中，通過加載相應(yīng)動態(tài)鏈接方法，保存起來的    /**      * ifn->CreateJavaVM =     *   (void *)GetProcAddress(handle, "JNI_CreateJavaVM");     * ifn->GetDefaultJavaVMInitArgs =     *   (void *)GetProcAddress(handle, "JNI_GetDefaultJavaVMInitArgs");     */    InvocationFunctions ifn = args->ifn;    JavaVM *vm = 0;    JNIEnv *env = 0;    jclass mainClass = NULL;    jclass appClass = NULL; // actual application class being launched    jmethodID mainID;    jobjectArray mainArgs;    int ret = 0;    jlong start, end;    // collector    RegisterThread();    /* Initialize the virtual machine */    start = CounterGet();    // 初始化jvm，失敗則退出    if (!InitializeJVM(&vm, &env, &ifn)) {        JLI_ReportErrorMessage(JVM_ERROR1);        exit(1);    }    // jvm檢查完畢，如果只是一些展示類請求，則展示信息后，退出jvm    if (showSettings != NULL) {        ShowSettings(env, showSettings);        /**         * 宏是神奇的操作，此處 *env 直接引用#define CHECK_EXCEPTION_LEAVE(CEL_return_value) \    do { \        if ((*env)->ExceptionOccurred(env)) { \            JLI_ReportExceptionDescription(env); \            ret = (CEL_return_value); \            LEAVE(); \        } \    } while (JNI_FALSE)         */        CHECK_EXCEPTION_LEAVE(1);    }    // 調(diào)用 LEAVE() 方法的目的在于主動銷毀jvm線程    // 且退出當(dāng)前方法調(diào)用，即 LEAVE() 后方法不再被執(zhí)行/* * Always detach the main thread so that it appears to have ended when * the application's main method exits.  This will invoke the * uncaught exception handler machinery if main threw an * exception.  An uncaught exception handler cannot change the * launcher's return code except by calling System.exit. * * Wait for all non-daemon threads to end, then destroy the VM. * This will actually create a trivial new Java waiter thread * named "DestroyJavaVM", but this will be seen as a different * thread from the one that executed main, even though they are * the same C thread.  This allows mainThread.join() and * mainThread.isAlive() to work as expected. */    /**     *     * #define LEAVE() \    do { \        if ((*vm)->DetachCurrentThread(vm) != JNI_OK) { \            JLI_ReportErrorMessage(JVM_ERROR2); \            ret = 1; \        } \        if (JNI_TRUE) { \            (*vm)->DestroyJavaVM(vm); \            return ret; \        } \    } while (JNI_FALSE)     */    if (printVersion || showVersion) {        PrintJavaVersion(env, showVersion);        CHECK_EXCEPTION_LEAVE(0);        if (printVersion) {            LEAVE();        }    }    /* If the user specified neither a class name nor a JAR file */    if (printXUsage || printUsage || what == 0 || mode == LM_UNKNOWN) {        PrintUsage(env, printXUsage);        CHECK_EXCEPTION_LEAVE(1);        LEAVE();    }    // 釋放內(nèi)存    FreeKnownVMs();  /* after last possible PrintUsage() */    if (JLI_IsTraceLauncher()) {        end = CounterGet();        JLI_TraceLauncher("%ld micro seconds to InitializeJVM\n",               (long)(jint)Counter2Micros(end-start));    }    /* At this stage, argc/argv have the application's arguments */    if (JLI_IsTraceLauncher()){        int i;        printf("%s is '%s'\n", launchModeNames[mode], what);        printf("App's argc is %d\n", argc);        for (i=0; i < argc; i++) {            printf("    argv[%2d] = '%s'\n", i, argv[i]);        }    }    ret = 1;    /*     * Get the application's main class.     *     * See bugid 5030265.  The Main-Class name has already been parsed     * from the manifest, but not parsed properly for UTF-8 support.     * Hence the code here ignores the value previously extracted and     * uses the pre-existing code to reextract the value.  This is     * possibly an end of release cycle expedient.  However, it has     * also been discovered that passing some character sets through     * the environment has "strange" behavior on some variants of     * Windows.  Hence, maybe the manifest parsing code local to the     * launcher should never be enhanced.     *     * Hence, future work should either:     *     1)   Correct the local parsing code and verify that the     *          Main-Class attribute gets properly passed through     *          all environments,     *     2)   Remove the vestages of maintaining main_class through     *          the environment (and remove these comments).     *     * This method also correctly handles launching existing JavaFX     * applications that may or may not have a Main-Class manifest entry.     */    // 加載 main 指定的class類    mainClass = LoadMainClass(env, mode, what);    CHECK_EXCEPTION_NULL_LEAVE(mainClass);    /*     * In some cases when launching an application that needs a helper, e.g., a     * JavaFX application with no main method, the mainClass will not be the     * applications own main class but rather a helper class. To keep things     * consistent in the UI we need to track and report the application main class.     */    appClass = GetApplicationClass(env);    NULL_CHECK_RETURN_VALUE(appClass, -1);    /*     * PostJVMInit uses the class name as the application name for GUI purposes,     * for example, on OSX this sets the application name in the menu bar for     * both SWT and JavaFX. So we'll pass the actual application class here     * instead of mainClass as that may be a launcher or helper class instead     * of the application class.     */    // 加載main() 方法前執(zhí)行初始化    PostJVMInit(env, appClass, vm);    CHECK_EXCEPTION_LEAVE(1);    /*     * The LoadMainClass not only loads the main class, it will also ensure     * that the main method's signature is correct, therefore further checking     * is not required. The main method is invoked here so that extraneous java     * stacks are not in the application stack trace.     */    // 獲取main()方法id, main(String[] args)    mainID = (*env)->GetStaticMethodID(env, mainClass, "main",                                       "([Ljava/lang/String;)V");    CHECK_EXCEPTION_NULL_LEAVE(mainID);    /* Build platform specific argument array */    // 構(gòu)建args[] 參數(shù)    mainArgs = CreateApplicationArgs(env, argv, argc);    CHECK_EXCEPTION_NULL_LEAVE(mainArgs);    /* Invoke main method. */    // 調(diào)用java實(shí)現(xiàn)的main()方法    // XX:: 重要實(shí)現(xiàn)    (*env)->CallStaticVoidMethod(env, mainClass, mainID, mainArgs);    /*     * The launcher's exit code (in the absence of calls to     * System.exit) will be non-zero if main threw an exception.     */    ret = (*env)->ExceptionOccurred(env) == NULL ? 0 : 1;    LEAVE();}/* * Loads a class and verifies that the main class is present and it is ok to * call it for more details refer to the java implementation. */static jclassLoadMainClass(JNIEnv *env, int mode, char *name){    jmethodID mid;    jstring str;    jobject result;    jlong start, end;    jclass cls = GetLauncherHelperClass(env);    NULL_CHECK0(cls);    if (JLI_IsTraceLauncher()) {        start = CounterGet();    }    // checkAndLoadMain(String) 方法作為中間main()調(diào)用    NULL_CHECK0(mid = (*env)->GetStaticMethodID(env, cls,                "checkAndLoadMain",                "(ZILjava/lang/String;)Ljava/lang/Class;"));    str = NewPlatformString(env, name);    CHECK_JNI_RETURN_0(        result = (*env)->CallStaticObjectMethod(            env, cls, mid, USE_STDERR, mode, str));    if (JLI_IsTraceLauncher()) {        end   = CounterGet();        printf("%ld micro seconds to load main class\n",               (long)(jint)Counter2Micros(end-start));        printf("----%s----\n", JLDEBUG_ENV_ENTRY);    }    return (jclass)result;}    
// 初始化jvm, 主要是調(diào)用 CreateJavaVM() 方法，進(jìn)行創(chuàng)建jvm操作/* * Initializes the Java Virtual Machine. Also frees options array when * finished. */static jbooleanInitializeJVM(JavaVM **pvm, JNIEnv **penv, InvocationFunctions *ifn){    JavaVMInitArgs args;    jint r;    memset(&args, 0, sizeof(args));    args.version  = JNI_VERSION_1_2;    args.nOptions = numOptions;    args.options  = options;    args.ignoreUnrecognized = JNI_FALSE;    if (JLI_IsTraceLauncher()) {        int i = 0;        printf("JavaVM args:\n    ");        printf("version 0x%08lx, ", (long)args.version);        printf("ignoreUnrecognized is %s, ",               args.ignoreUnrecognized ? "JNI_TRUE" : "JNI_FALSE");        printf("nOptions is %ld\n", (long)args.nOptions);        for (i = 0; i < numOptions; i++)            printf("    option[%2d] = '%s'\n",                   i, args.options[i].optionString);    }    r = ifn->CreateJavaVM(pvm, (void **)penv, &args);    JLI_MemFree(options);    return r == JNI_OK;}

// share/bin/java.c/* * Entry point. */intJLI_Launch(int argc, char ** argv,              /* main argc, argc */        int jargc, const char** jargv,          /* java args */        int appclassc, const char** appclassv,  /* app classpath */        const char* fullversion,                /* full version defined */        const char* dotversion,                 /* dot version defined */        const char* pname,                      /* program name */        const char* lname,                      /* launcher name */        jboolean javaargs,                      /* JAVA_ARGS */        jboolean cpwildcard,                    /* classpath wildcard*/        jboolean javaw,                         /* windows-only javaw */        jint ergo                               /* ergonomics class policy */){    int mode = LM_UNKNOWN;    char *what = NULL;    char *cpath = 0;    char *main_class = NULL;    int ret;    InvocationFunctions ifn;    jlong start, end;    char jvmpath[MAXPATHLEN];    char jrepath[MAXPATHLEN];    char jvmcfg[MAXPATHLEN];    _fVersion = fullversion;    _dVersion = dotversion;    _launcher_name = lname;    _program_name = pname;    _is_java_args = javaargs;    _wc_enabled = cpwildcard;    _ergo_policy = ergo;    // 初始化啟動器    InitLauncher(javaw);    // 打印狀態(tài)    DumpState();    // 跟蹤調(diào)用啟動    if (JLI_IsTraceLauncher()) {        int i;        printf("Command line args:\n");        for (i = 0; i < argc ; i++) {            printf("argv[%d] = %s\n", i, argv[i]);        }        AddOption("-Dsun.java.launcher.diag=true", NULL);    }    /*     * Make sure the specified version of the JRE is running.     *     * There are three things to note about the SelectVersion() routine:     *  1) If the version running isn't correct, this routine doesn't     *     return (either the correct version has been exec'd or an error     *     was issued).     *  2) Argc and Argv in this scope are *not* altered by this routine.     *     It is the responsibility of subsequent code to ignore the     *     arguments handled by this routine.     *  3) As a side-effect, the variable "main_class" is guaranteed to     *     be set (if it should ever be set).  This isn't exactly the     *     poster child for structured programming, but it is a small     *     price to pay for not processing a jar file operand twice.     *     (Note: This side effect has been disabled.  See comment on     *     bugid 5030265 below.)     */    // 解析命令行參數(shù)，選擇一jre版本    SelectVersion(argc, argv, &main_class);    CreateExecutionEnvironment(&argc, &argv,                               jrepath, sizeof(jrepath),                               jvmpath, sizeof(jvmpath),                               jvmcfg,  sizeof(jvmcfg));    if (!IsJavaArgs()) {        // 設(shè)置一些特殊的環(huán)境變量        SetJvmEnvironment(argc,argv);    }    ifn.CreateJavaVM = 0;    ifn.GetDefaultJavaVMInitArgs = 0;    if (JLI_IsTraceLauncher()) {        start = CounterGet();    }    // 加載VM, 重中之重    if (!LoadJavaVM(jvmpath, &ifn)) {        return(6);    }    if (JLI_IsTraceLauncher()) {        end   = CounterGet();    }    JLI_TraceLauncher("%ld micro seconds to LoadJavaVM\n",             (long)(jint)Counter2Micros(end-start));    ++argv;    --argc;    // 解析更多參數(shù)信息    if (IsJavaArgs()) {        /* Preprocess wrapper arguments */        TranslateApplicationArgs(jargc, jargv, &argc, &argv);        if (!AddApplicationOptions(appclassc, appclassv)) {            return(1);        }    } else {        /* Set default CLASSPATH */        cpath = getenv("CLASSPATH");        if (cpath == NULL) {            cpath = ".";        }        SetClassPath(cpath);    }    /* Parse command line options; if the return value of     * ParseArguments is false, the program should exit.     */    // 解析參數(shù)    if (!ParseArguments(&argc, &argv, &mode, &what, &ret, jrepath))    {        return(ret);    }    /* Override class path if -jar flag was specified */    if (mode == LM_JAR) {        SetClassPath(what);     /* Override class path */    }    /* set the -Dsun.java.command pseudo property */    SetJavaCommandLineProp(what, argc, argv);    /* Set the -Dsun.java.launcher pseudo property */    SetJavaLauncherProp();    /* set the -Dsun.java.launcher.* platform properties */    SetJavaLauncherPlatformProps();    return JVMInit(&ifn, threadStackSize, argc, argv, mode, what, ret);}
// macos/bin/java_md_macos.c// MacOSX we may continue in the same threadintJVMInit(InvocationFunctions* ifn, jlong threadStackSize,                 int argc, char **argv,                 int mode, char *what, int ret) {    if (sameThread) {        JLI_TraceLauncher("In same thread\n");        // need to block this thread against the main thread        // so signals get caught correctly        __block int rslt = 0;        NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];        {            NSBlockOperation *op = [NSBlockOperation blockOperationWithBlock: ^{                JavaMainArgs args;                args.argc = argc;                args.argv = argv;                args.mode = mode;                args.what = what;                args.ifn  = *ifn;                // 調(diào)用 JavaMain()                rslt = JavaMain(&args);            }];            /*             * We cannot use dispatch_sync here, because it blocks the main dispatch queue.             * Using the main NSRunLoop allows the dispatch queue to run properly once             * SWT (or whatever toolkit this is needed for) kicks off it's own NSRunLoop             * and starts running.             */            [op performSelectorOnMainThread:@selector(start) withObject:nil waitUntilDone:YES];        }        [pool drain];        return rslt;    } else {        return ContinueInNewThread(ifn, threadStackSize, argc, argv, mode, what, ret);    }}

// windows/bin/java_md.cintJVMInit(InvocationFunctions* ifn, jlong threadStackSize,        int argc, char **argv,        int mode, char *what, int ret){    ShowSplashScreen();    return ContinueInNewThread(ifn, threadStackSize, argc, argv, mode, what, ret);}// java.cintContinueInNewThread(InvocationFunctions* ifn, jlong threadStackSize,                    int argc, char **argv,                    int mode, char *what, int ret){
    /*     * If user doesn't specify stack size, check if VM has a preference.     * Note that HotSpot no longer supports JNI_VERSION_1_1 but it will     * return its default stack size through the init args structure.     */    if (threadStackSize == 0) {      struct JDK1_1InitArgs args1_1;      memset((void*)&args1_1, 0, sizeof(args1_1));      args1_1.version = JNI_VERSION_1_1;      ifn->GetDefaultJavaVMInitArgs(&args1_1);  /* ignore return value */      if (args1_1.javaStackSize > 0) {         threadStackSize = args1_1.javaStackSize;      }    }
    { /* Create a new thread to create JVM and invoke main method */      JavaMainArgs args;      int rslt;
      args.argc = argc;      args.argv = argv;      args.mode = mode;      args.what = what;      args.ifn = *ifn;      // 傳入 JavaMain() 函數(shù)參數(shù)，在新線程中運(yùn)行 JavaMain      rslt = ContinueInNewThread0(JavaMain, threadStackSize, (void*)&args);      /* If the caller has deemed there is an error we       * simply return that, otherwise we return the value of       * the callee       */      return (ret != 0) ? ret : rslt;    }}/* * Block current thread and continue execution in a new thread */intContinueInNewThread0(int (JNICALL *continuation)(void *), jlong stack_size, void * args) {    int rslt = 0;    unsigned thread_id;#ifndef STACK_SIZE_PARAM_IS_A_RESERVATION#define STACK_SIZE_PARAM_IS_A_RESERVATION  (0x10000)#endif    /*     * STACK_SIZE_PARAM_IS_A_RESERVATION is what we want, but it's not     * supported on older version of Windows. Try first with the flag; and     * if that fails try again without the flag. See MSDN document or HotSpot     * source (os_win32.cpp) for details.     * 調(diào)用底層內(nèi)核方法，創(chuàng)建新線程，將JavaMain(args)作為函數(shù)調(diào)用處理。創(chuàng)建后立即執(zhí)行     * 如果第一次帶參創(chuàng)建失敗，則棄參重試一次     */    HANDLE thread_handle =      (HANDLE)_beginthreadex(NULL,                             (unsigned)stack_size,                             continuation,                             args,                             STACK_SIZE_PARAM_IS_A_RESERVATION,                             &thread_id);    if (thread_handle == NULL) {      thread_handle =      (HANDLE)_beginthreadex(NULL,                             (unsigned)stack_size,                             continuation,                             args,                             0,                             &thread_id);    }    /* AWT preloading (AFTER main thread start) */#ifdef ENABLE_AWT_PRELOAD    /* D3D preloading */    if (awtPreloadD3D != 0) {        char *envValue;        /* D3D routines checks env.var J2D_D3D if no appropriate         * command line params was specified         */        envValue = getenv("J2D_D3D");        if (envValue != NULL && JLI_StrCaseCmp(envValue, "false") == 0) {            awtPreloadD3D = 0;        }        /* Test that AWT preloading isn't disabled by J2D_D3D_PRELOAD env.var */        envValue = getenv("J2D_D3D_PRELOAD");        if (envValue != NULL && JLI_StrCaseCmp(envValue, "false") == 0) {            awtPreloadD3D = 0;        }        if (awtPreloadD3D < 0) {            /* If awtPreloadD3D is still undefined (-1), test             * if it is turned on by J2D_D3D_PRELOAD env.var.             * By default it's turned OFF.             */            awtPreloadD3D = 0;            if (envValue != NULL && JLI_StrCaseCmp(envValue, "true") == 0) {                awtPreloadD3D = 1;            }         }    }    if (awtPreloadD3D) {        AWTPreload(D3D_PRELOAD_FUNC);    }#endif /* ENABLE_AWT_PRELOAD */    if (thread_handle) {      // 等待線程結(jié)束，并獲取返回碼      WaitForSingleObject(thread_handle, INFINITE);      GetExitCodeThread(thread_handle, &rslt);      CloseHandle(thread_handle);    } else {      // 如果實(shí)在是創(chuàng)建線程失敗，則自身直接執(zhí)行該方法即可      rslt = continuation(args);    }#ifdef ENABLE_AWT_PRELOAD    if (awtPreloaded) {        AWTPreloadStop();    }#endif /* ENABLE_AWT_PRELOAD */    return rslt;}

// solaris/bin/java_md_solinux.c/* * Block current thread and continue execution in a new thread */intContinueInNewThread0(int (JNICALL *continuation)(void *), jlong stack_size, void * args) {    int rslt;#ifdef __linux__    pthread_t tid;    pthread_attr_t attr;    pthread_attr_init(&attr);    pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
    if (stack_size > 0) {      pthread_attr_setstacksize(&attr, stack_size);    }    // 常見的 pthread_xx 方式 創(chuàng)建線程    if (pthread_create(&tid, &attr, (void *(*)(void*))continuation, (void*)args) == 0) {      void * tmp;      pthread_join(tid, &tmp);      rslt = (int)tmp;    } else {     /*      * Continue execution in current thread if for some reason (e.g. out of      * memory/LWP)  a new thread can't be created. This will likely fail      * later in continuation as JNI_CreateJavaVM needs to create quite a      * few new threads, anyway, just give it a try..      */      rslt = continuation(args);    }
    pthread_attr_destroy(&attr);#else /* ! __linux__ */    thread_t tid;    long flags = 0;    if (thr_create(NULL, stack_size, (void *(*)(void *))continuation, args, flags, &tid) == 0) {      void * tmp;      thr_join(tid, NULL, &tmp);      rslt = (int)tmp;    } else {      /* See above. Continue in current thread if thr_create() failed */      rslt = continuation(args);    }#endif /* __linux__ */    return rslt;}