JVM的启动流程

作为Java开发者,一般都是直接使用java命令来运行Java代码,但一直没有想过java命令的背后做了哪些工作。其实从java命令到Java中的主方法之间还做了很多其他重要操作,比如创建和初始化JVM。本文就来大体分析一下这个过程是怎样的。


本文仅会分析JVM启动的主干流程,即从java命令的main函数到Java主类的main方法,对于其他分支中的细节不会过多分析。

main函数

c
jdk/src/share/bin/main.c
// HotSpot VM的启动入口
int
main(int argc, char **argv)
{
    int margc;
    char** margv;
    const jboolean const_javaw = JNI_FALSE;
#endif /* JAVAW */
#ifdef _WIN32
    {
        int i = 0;
        if (getenv(JLDEBUG_ENV_ENTRY) != NULL) {
            printf("Windows original main args:\n");
            for (i = 0 ; i < __argc ; i++) {
                printf("wwwd_args[%d] = %s\n", i, __argv[i]);
            }
        }
    }
    JLI_CmdToArgs(GetCommandLine());
    margc = JLI_GetStdArgc();
    // add one more to mark the end
    margv = (char **)JLI_MemAlloc((margc + 1) * (sizeof(char *)));
    {
        int i = 0;
        StdArg *stdargs = JLI_GetStdArgs();
        for (i = 0 ; i < margc ; i++) {
            margv[i] = stdargs[i].arg;
        }
        margv[i] = NULL;
    }
#else /* *NIXES */
    margc = argc;
    margv = argv;
#endif /* WIN32 */
    // java launcher
    return JLI_Launch(margc, margv,
                   sizeof(const_jargs) / sizeof(char *), const_jargs,
                   sizeof(const_appclasspath) / sizeof(char *), const_appclasspath,
                   FULL_VERSION,
                   DOT_VERSION,
                   (const_progname != NULL) ? const_progname : *margv,
                   (const_launcher != NULL) ? const_launcher : *margv,
                   (const_jargs != NULL) ? JNI_TRUE : JNI_FALSE,
                   const_cpwildcard, const_javaw, const_ergo_class);
}

JLI_Launch函数

c
jdk/src/share/bin/java.c
int
JLI_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;
    /* 保存入口函数信息 */
    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;

    InitLauncher(javaw);
    DumpState();
    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.)
     */
    SelectVersion(argc, argv, &main_class);

    CreateExecutionEnvironment(&argc, &argv,
                               jrepath, sizeof(jrepath),
                               jvmpath, sizeof(jvmpath),
                               jvmcfg,  sizeof(jvmcfg));

    ifn.CreateJavaVM = 0;
    ifn.GetDefaultJavaVMInitArgs = 0;

    if (JLI_IsTraceLauncher()) {
        start = CounterGet();
    }

    /*
     * 加载libjvm.so动态链接库
     * jvmpath: /openjdk/build/linux-x86_64-normal-server-slowdebug/jdk/lib/amd64/server/libjvm.so
     * ifn: InvocationFunctions类型变量
     */
    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;

    if (IsJavaArgs()) {
        /* Preprocess wrapper arguments */
        TranslateApplicationArgs(jargc, jargv, &argc, &argv);
        if (!AddApplicationOptions(appclassc, appclassv)) {
            return(1);
        }
    } else {
        /* Set default CLASSPATH */
        // 从环境变量中获取类路径
        cpath = getenv("CLASSPATH");
        if (cpath == NULL) {
            // 设置默认的类路径
            cpath = ".";
        }
        // 设置类路径
        SetClassPath(cpath);
    }

    /* Parse command line options; if the return value of
     * ParseArguments is false, the program should exit.
     */
    /*
     * 解析参数
     */
    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();

    // 初始化JVM
    return JVMInit(&ifn, threadStackSize, argc, argv, mode, what, ret);
}

这个函数中重点关注下面几部分:

  • 加载libjvm.so动态链接库;
  • 解析参数;
  • 初始化JVM(主流程);

加载动态链接库

c
jdk/src/solaris/bin/java_md_solinux.c
jboolean
LoadJavaVM(const char *jvmpath, InvocationFunctions *ifn)
{
    void *libjvm;

    JLI_TraceLauncher("JVM path is %s\n", jvmpath);

    // 打开动态链接库
    libjvm = dlopen(jvmpath, RTLD_NOW + RTLD_GLOBAL);
    if (libjvm == NULL) {
#if defined(__solaris__) && defined(__sparc) && !defined(_LP64) /* i.e. 32-bit sparc */
      FILE * fp;
      Elf32_Ehdr elf_head;
      int count;
      int location;

      fp = fopen(jvmpath, "r");
      if (fp == NULL) {
        JLI_ReportErrorMessage(DLL_ERROR2, jvmpath, dlerror());
        return JNI_FALSE;
      }

      /* read in elf header */
      count = fread((void*)(&elf_head), sizeof(Elf32_Ehdr), 1, fp);
      fclose(fp);
      if (count < 1) {
        JLI_ReportErrorMessage(DLL_ERROR2, jvmpath, dlerror());
        return JNI_FALSE;
      }

      /*
       * Check for running a server vm (compiled with -xarch=v8plus)
       * on a stock v8 processor.  In this case, the machine type in
       * the elf header would not be included the architecture list
       * provided by the isalist command, which is turn is gotten from
       * sysinfo.  This case cannot occur on 64-bit hardware and thus
       * does not have to be checked for in binaries with an LP64 data
       * model.
       */
      if (elf_head.e_machine == EM_SPARC32PLUS) {
        char buf[257];  /* recommended buffer size from sysinfo man
                           page */
        long length;
        char* location;

        length = sysinfo(SI_ISALIST, buf, 257);
        if (length > 0) {
            location = JLI_StrStr(buf, "sparcv8plus ");
          if (location == NULL) {
            JLI_ReportErrorMessage(JVM_ERROR3);
            return JNI_FALSE;
          }
        }
      }
#endif
        JLI_ReportErrorMessage(DLL_ERROR1, __LINE__);
        JLI_ReportErrorMessage(DLL_ERROR2, jvmpath, dlerror());
        return JNI_FALSE;
    }

    // 设置创建JVM的函数,后续会调用
    ifn->CreateJavaVM = (CreateJavaVM_t)
        dlsym(libjvm, "JNI_CreateJavaVM");
    if (ifn->CreateJavaVM == NULL) {
        JLI_ReportErrorMessage(DLL_ERROR2, jvmpath, dlerror());
        return JNI_FALSE;
    }

    ifn->GetDefaultJavaVMInitArgs = (GetDefaultJavaVMInitArgs_t)
        dlsym(libjvm, "JNI_GetDefaultJavaVMInitArgs");
    if (ifn->GetDefaultJavaVMInitArgs == NULL) {
        JLI_ReportErrorMessage(DLL_ERROR2, jvmpath, dlerror());
        return JNI_FALSE;
    }

    ifn->GetCreatedJavaVMs = (GetCreatedJavaVMs_t)
        dlsym(libjvm, "JNI_GetCreatedJavaVMs");
    if (ifn->GetCreatedJavaVMs == NULL) {
        JLI_ReportErrorMessage(DLL_ERROR2, jvmpath, dlerror());
        return JNI_FALSE;
    }

    return JNI_TRUE;
}

这里会加载libjvm.so动态链接库,并把一些关键函数设置到ifn中,后续会调用。

解析参数

c
jdk/src/share/bin/java.c
static jboolean
ParseArguments(int *pargc, char ***pargv,
               int *pmode, char **pwhat,
               int *pret, const char *jrepath)
{
    int argc = *pargc;
    char **argv = *pargv;
    int mode = LM_UNKNOWN;
    char *arg;

    *pret = 0;

    /*
     * 选项参数一定要在类名参数前面,如果把主类类名放在第一个,那么这里根本进不去循环。
     */
    while ((arg = *argv) != 0 && *arg == '-') {
        argv++; --argc;
        if (JLI_StrCmp(arg, "-classpath") == 0 || JLI_StrCmp(arg, "-cp") == 0) {
            ARG_CHECK (argc, ARG_ERROR1, arg);
            SetClassPath(*argv);
            mode = LM_CLASS;
            argv++; --argc;
        } else if (JLI_StrCmp(arg, "-jar") == 0) {
            ARG_CHECK (argc, ARG_ERROR2, arg);
            mode = LM_JAR;
        } else if (JLI_StrCmp(arg, "-help") == 0 ||
                   JLI_StrCmp(arg, "-h") == 0 ||
                   JLI_StrCmp(arg, "-?") == 0) {
            printUsage = JNI_TRUE;
            return JNI_TRUE;
        } else if (JLI_StrCmp(arg, "-version") == 0) {
            printVersion = JNI_TRUE;
            return JNI_TRUE;
        } else if (JLI_StrCmp(arg, "-showversion") == 0) {
            showVersion = JNI_TRUE;
        } else if (JLI_StrCmp(arg, "-X") == 0) {
            printXUsage = JNI_TRUE;
            return JNI_TRUE;
/*
 * The following case checks for -XshowSettings OR -XshowSetting:SUBOPT.
 * In the latter case, any SUBOPT value not recognized will default to "all"
 */
        } else if (JLI_StrCmp(arg, "-XshowSettings") == 0 ||
                JLI_StrCCmp(arg, "-XshowSettings:") == 0) {
            showSettings = arg;
        } else if (JLI_StrCmp(arg, "-Xdiag") == 0) {
            AddOption("-Dsun.java.launcher.diag=true", NULL);
/*
 * The following case provide backward compatibility with old-style
 * command line options.
 */
        } else if (JLI_StrCmp(arg, "-fullversion") == 0) {
            JLI_ReportMessage("%s full version \"%s\"", _launcher_name, GetFullVersion());
            return JNI_FALSE;
        } else if (JLI_StrCmp(arg, "-verbosegc") == 0) {
            AddOption("-verbose:gc", NULL);
        } else if (JLI_StrCmp(arg, "-t") == 0) {
            AddOption("-Xt", NULL);
        } else if (JLI_StrCmp(arg, "-tm") == 0) {
            AddOption("-Xtm", NULL);
        } else if (JLI_StrCmp(arg, "-debug") == 0) {
            AddOption("-Xdebug", NULL);
        } else if (JLI_StrCmp(arg, "-noclassgc") == 0) {
            AddOption("-Xnoclassgc", NULL);
        } else if (JLI_StrCmp(arg, "-Xfuture") == 0) {
            AddOption("-Xverify:all", NULL);
        } else if (JLI_StrCmp(arg, "-verify") == 0) {
            AddOption("-Xverify:all", NULL);
        } else if (JLI_StrCmp(arg, "-verifyremote") == 0) {
            AddOption("-Xverify:remote", NULL);
        } else if (JLI_StrCmp(arg, "-noverify") == 0) {
            AddOption("-Xverify:none", NULL);
        } else if (JLI_StrCCmp(arg, "-prof") == 0) {
            char *p = arg + 5;
            char *tmp = JLI_MemAlloc(JLI_StrLen(arg) + 50);
            if (*p) {
                sprintf(tmp, "-Xrunhprof:cpu=old,file=%s", p + 1);
            } else {
                sprintf(tmp, "-Xrunhprof:cpu=old,file=java.prof");
            }
            AddOption(tmp, NULL);
        } else if (JLI_StrCCmp(arg, "-ss") == 0 ||
                   JLI_StrCCmp(arg, "-oss") == 0 ||
                   JLI_StrCCmp(arg, "-ms") == 0 ||
                   JLI_StrCCmp(arg, "-mx") == 0) {
            char *tmp = JLI_MemAlloc(JLI_StrLen(arg) + 6);
            sprintf(tmp, "-X%s", arg + 1); /* skip '-' */
            AddOption(tmp, NULL);
        } else if (JLI_StrCmp(arg, "-checksource") == 0 ||
                   JLI_StrCmp(arg, "-cs") == 0 ||
                   JLI_StrCmp(arg, "-noasyncgc") == 0) {
            /* No longer supported */
            JLI_ReportErrorMessage(ARG_WARN, arg);
        } else if (JLI_StrCCmp(arg, "-version:") == 0 ||
                   JLI_StrCmp(arg, "-no-jre-restrict-search") == 0 ||
                   JLI_StrCmp(arg, "-jre-restrict-search") == 0 ||
                   JLI_StrCCmp(arg, "-splash:") == 0) {
            ; /* Ignore machine independent options already handled */
        } else if (ProcessPlatformOption(arg)) {
            ; /* Processing of platform dependent options */
        } else if (RemovableOption(arg)) {
            ; /* Do not pass option to vm. */
        } else {
            AddOption(arg, NULL);
        }
    }

    if (--argc >= 0) {
        *pwhat = *argv++;
    }

    if (*pwhat == NULL) {
        *pret = 1;
    } else if (mode == LM_UNKNOWN) {
        /* default to LM_CLASS if -jar and -cp option are
         * not specified */
        mode = LM_CLASS;
    }

    if (argc >= 0) {
        *pargc = argc;
        *pargv = argv;
    }

    *pmode = mode;

    return JNI_TRUE;
}

有一点非常重要,在使用java命令执行Java主类时,要确保参数或选项要放在java命令和主类之间,也就是说主类要放在最后,主类后面的参数是无法被加载到的。

初始化JVM

c
jdk/src/solaris/bin/java_md_solinux.c
int
JVMInit(InvocationFunctions* ifn, jlong threadStackSize,
        int argc, char **argv,
        int mode, char *what, int ret)
{
    ShowSplashScreen();
    // 根据方法名可以看出,在新线程中继续执行
    return ContinueInNewThread(ifn, threadStackSize, argc, argv, mode, what, ret);
}

主要实现在ContinueInNewThread函数中。

c
jdk/src/share/bin/java.c
int
ContinueInNewThread(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) {
          // 设置栈大小
         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;

      // 调用ContinueInNewThread0()函数创建一个JVM实例并执行Java主类的main()方法
      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;
    }
}

继续跟踪ContinueInNewThread0函数。

c
jdk/src/solaris/bin/java_md_solinux.c
int
ContinueInNewThread0(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);
    }

    // 创建新线程,并让新线程执行JavaMain主函数
    if (pthread_create(&tid, &attr, (void *(*)(void*))continuation, (void*)args) == 0) {
      void * tmp;
      // 当前线程hi阻塞在这里
      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;
}

在该函数中,调用glibc中的pthread线程库创建了一个新的线程,并以continuation参数作为新线程的入口函数。那这个参数是什么值呢?在ContinueInNewThread函数中传进来的,就是JavaMain函数。

从这里也可以看出来执行java命令的线程并不是最终执行我们编写的Java主方法的线程。

JavaMain函数

c
jdk/src/share/bin/java.c
int JNICALL
JavaMain(void * _args)
{

    printf("Welcome to lzip-jdk8!\n");

    JavaMainArgs *args = (JavaMainArgs *)_args;
    int argc = args->argc;
    char **argv = args->argv;
    int mode = args->mode;
    char *what = args->what;
    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;

    RegisterThread();

    /* Initialize the virtual machine */
    start = CounterGet();
    /* 
     * InitializeJVM()函数初始化JVM,给JavaVM和JNIEnv对象正确赋值,通过调用
     * InvocationFunctions结构体下的CreateJavaVM()函数指针来实现,该指针在
     * LoadJavaVM()函数中指向libjvm.so动态链接库中的JNI_CreateJavaVM()函数,具体实现在jni.cpp文件。
     */
    if (!InitializeJVM(&vm, &env, &ifn)) {
        JLI_ReportErrorMessage(JVM_ERROR1);
        exit(1);
    }

    if (showSettings != NULL) {
        ShowSettings(env, showSettings);
        CHECK_EXCEPTION_LEAVE(1);
    }

    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();
    }

    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.
     */
    /*
     * 加载Java主类
     */
    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.
     */
    // jvm初始化后操作
    PostJVMInit(env, appClass, vm);
    /*
     * 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.
     */
    // 从Java主类中查找main()方法对应的唯一ID
    mainID = (*env)->GetStaticMethodID(env, mainClass, "main",
                                       "([Ljava/lang/String;)V");
    CHECK_EXCEPTION_NULL_LEAVE(mainID);

    /* Build platform specific argument array */
    // 创建针对平台的参数数组
    mainArgs = CreateApplicationArgs(env, argv, argc);
    CHECK_EXCEPTION_NULL_LEAVE(mainArgs);

    /* Invoke main method. */
    /*
     * ========================
     *  调用Java主类的main()方法
     * ========================
     *  下面被调用的方法的具体实现在hotspot/src/share/vm/prims/jni.cpp文件中
     */
    (*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();
}

关注该函数中的重要步骤:

  • 初始化JVM;
  • 加载主类;
  • 执行主类中的main方法;

初始化JVM

c
jdk/src/share/bin/java.c
static jboolean
InitializeJVM(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);
    }

    // 参考jni.cpp中的JNI_CreateJavaVM
    r = ifn->CreateJavaVM(pvm, (void **)penv, &args);
    JLI_MemFree(options);
    return r == JNI_OK;
}

注意,虽然调用的CreateJavaVM函数叫创建JVM,但实际上内部包含了很多初始化操作。实际调用到的函数是在jni.cpp文件中,该函数非常长,执行了很多模块的初始化,

JNI_CreateJavaVM
create_vm
<
>
cpp
hotspot/src/share/vm/prims/jni.cpp
_JNI_IMPORT_OR_EXPORT_ jint JNICALL JNI_CreateJavaVM(JavaVM **vm, void **penv, void *args) {
#ifndef USDT2
  HS_DTRACE_PROBE3(hotspot_jni, CreateJavaVM__entry, vm, penv, args);
#else /* USDT2 */
  HOTSPOT_JNI_CREATEJAVAVM_ENTRY(
                                 (void **) vm, penv, args);
#endif /* USDT2 */

  jint result = JNI_ERR;
  DT_RETURN_MARK(CreateJavaVM, jint, (const jint&)result);

  // We're about to use Atomic::xchg for synchronization.  Some Zero
  // platforms use the GCC builtin __sync_lock_test_and_set for this,
  // but __sync_lock_test_and_set is not guaranteed to do what we want
  // on all architectures.  So we check it works before relying on it.
#if defined(ZERO) && defined(ASSERT)
  {
    jint a = 0xcafebabe;
    jint b = Atomic::xchg(0xdeadbeef, &a);
    void *c = &a;
    void *d = Atomic::xchg_ptr(&b, &c);
    assert(a == (jint) 0xdeadbeef && b == (jint) 0xcafebabe, "Atomic::xchg() works");
    assert(c == &b && d == &a, "Atomic::xchg_ptr() works");
  }
#endif // ZERO && ASSERT

  // At the moment it's only possible to have one Java VM,
  // since some of the runtime state is in global variables.

  // We cannot use our mutex locks here, since they only work on
  // Threads. We do an atomic compare and exchange to ensure only
  // one thread can call this method at a time

  // We use Atomic::xchg rather than Atomic::add/dec since on some platforms
  // the add/dec implementations are dependent on whether we are running
  // on a multiprocessor, and at this stage of initialization the os::is_MP
  // function used to determine this will always return false. Atomic::xchg
  // does not have this problem.
  if (Atomic::xchg(1, &vm_created) == 1) {
    return JNI_EEXIST;   // already created, or create attempt in progress
  }
  if (Atomic::xchg(0, &safe_to_recreate_vm) == 0) {
    return JNI_ERR;  // someone tried and failed and retry not allowed.
  }

  assert(vm_created == 1, "vm_created is true during the creation");

  /**
   * Certain errors during initialization are recoverable and do not
   * prevent this method from being called again at a later time
   * (perhaps with different arguments).  However, at a certain
   * point during initialization if an error occurs we cannot allow
   * this function to be called again (or it will crash).  In those
   * situations, the 'canTryAgain' flag is set to false, which atomically
   * sets safe_to_recreate_vm to 1, such that any new call to
   * JNI_CreateJavaVM will immediately fail using the above logic.
   */
  bool can_try_again = true;
  /*
   * 创建虚拟机
   */
  result = Threads::create_vm((JavaVMInitArgs*) args, &can_try_again);
  if (result == JNI_OK) {
    JavaThread *thread = JavaThread::current();
    /* thread is thread_in_vm here */
    *vm = (JavaVM *)(&main_vm);
    *(JNIEnv**)penv = thread->jni_environment();

    // Tracks the time application was running before GC
    RuntimeService::record_application_start();

    // Notify JVMTI
    if (JvmtiExport::should_post_thread_life()) {
       JvmtiExport::post_thread_start(thread);
    }

    EventThreadStart event;
    if (event.should_commit()) {
      event.set_javalangthread(java_lang_Thread::thread_id(thread->threadObj()));
      event.commit();
    }

#ifndef PRODUCT
  #ifndef TARGET_OS_FAMILY_windows
    #define CALL_TEST_FUNC_WITH_WRAPPER_IF_NEEDED(f) f()
  #endif

    // Check if we should compile all classes on bootclasspath
    if (CompileTheWorld) ClassLoader::compile_the_world();
    if (ReplayCompiles) ciReplay::replay(thread);

    // Some platforms (like Win*) need a wrapper around these test
    // functions in order to properly handle error conditions.
    CALL_TEST_FUNC_WITH_WRAPPER_IF_NEEDED(test_error_handler);
    CALL_TEST_FUNC_WITH_WRAPPER_IF_NEEDED(execute_internal_vm_tests);
#endif

    // Since this is not a JVM_ENTRY we have to set the thread state manually before leaving.
    ThreadStateTransition::transition_and_fence(thread, _thread_in_vm, _thread_in_native);
  } else {
    if (can_try_again) {
      // reset safe_to_recreate_vm to 1 so that retrial would be possible
      safe_to_recreate_vm = 1;
    }

    // Creation failed. We must reset vm_created
    *vm = 0;
    *(JNIEnv**)penv = 0;
    // reset vm_created last to avoid race condition. Use OrderAccess to
    // control both compiler and architectural-based reordering.
    OrderAccess::release_store(&vm_created, 0);
  }

  return result;
}
cpp
hotspot/src/share/vm/runtime/thread.cpp
/*
 * 创建虚拟机,名为创建,实则是在做初始化操作。
 * 为Java运行环境做准备。
 */
jint Threads::create_vm(JavaVMInitArgs* args, bool* canTryAgain) {

  extern void JDK_Version_init();

  // Check version
  if (!is_supported_jni_version(args->version)) return JNI_EVERSION;

  // Initialize the output stream module
  // 初始化输出流模块
  ostream_init();

  // Process java launcher properties.
  // 配置Launcher的属性
  Arguments::process_sun_java_launcher_properties(args);

  // Initialize the os module before using TLS
  // OS模块第一次初始化
  os::init();

  // Initialize system properties.
  // 配置系统属性
  Arguments::init_system_properties();

  // So that JDK version can be used as a discrimintor when parsing arguments
  JDK_Version_init();

  // Update/Initialize System properties after JDK version number is known
  // 配置特定于版本的系统属性
  Arguments::init_version_specific_system_properties();

  // Parse arguments
  jint parse_result = Arguments::parse(args);
  if (parse_result != JNI_OK) return parse_result;

  os::init_before_ergo();

  // 做一些前期工作,包括设置一些参数的默认值
  jint ergo_result = Arguments::apply_ergo();
  if (ergo_result != JNI_OK) return ergo_result;

  if (PauseAtStartup) {
    os::pause();
  }

#ifndef USDT2
  HS_DTRACE_PROBE(hotspot, vm__init__begin);
#else /* USDT2 */
  HOTSPOT_VM_INIT_BEGIN();
#endif /* USDT2 */

  // Record VM creation timing statistics
  TraceVmCreationTime create_vm_timer;
  create_vm_timer.start();

  // Timing (must come after argument parsing)
  TraceTime timer("Create VM", TraceStartupTime);

  // Initialize the os module after parsing the args
  // OS模块第二次初始化
  jint os_init_2_result = os::init_2();
  if (os_init_2_result != JNI_OK) return os_init_2_result;

  jint adjust_after_os_result = Arguments::adjust_after_os();
  if (adjust_after_os_result != JNI_OK) return adjust_after_os_result;

  // intialize TLS
  // 初始化线程本地存储
  ThreadLocalStorage::init();

  // Bootstrap native memory tracking, so it can start recording memory
  // activities before worker thread is started. This is the first phase
  // of bootstrapping, VM is currently running in single-thread mode.
  MemTracker::bootstrap_single_thread();

  // Initialize output stream logging
  ostream_init_log();

  // Convert -Xrun to -agentlib: if there is no JVM_OnLoad
  // Must be before create_vm_init_agents()
  if (Arguments::init_libraries_at_startup()) {
    convert_vm_init_libraries_to_agents();
  }

  // Launch -agentlib/-agentpath and converted -Xrun agents
  if (Arguments::init_agents_at_startup()) {
    create_vm_init_agents();
  }

  // Initialize Threads state
  _thread_list = NULL;
  _number_of_threads = 0;
  _number_of_non_daemon_threads = 0;

  // Initialize global data structures and create system classes in heap
  /*
   * 初始化全局数据结构
   */
  vm_init_globals();

  // Attach the main thread to this os thread
  /*
   * 创建代表Java主线程的JavaThread对象
   */
  JavaThread* main_thread = new JavaThread();
  // 设置主线程状态,表示正处于JVM中执行的状态
  main_thread->set_thread_state(_thread_in_vm);
  // must do this before set_active_handles and initialize_thread_local_storage
  // Note: on solaris initialize_thread_local_storage() will (indirectly)
  // change the stack size recorded here to one based on the java thread
  // stacksize. This adjusted size is what is used to figure the placement
  // of the guard pages.
  // 记录线程栈的基址和大小
  main_thread->record_stack_base_and_size();
  // 初始化线程本地存储区(TLS)
  main_thread->initialize_thread_local_storage();
  // 为线程设置JNI句柄
  main_thread->set_active_handles(JNIHandleBlock::allocate_block());

  // set_as_starting_thread()方法中会通过OS模块创建操作系统级别的(主)线程,上面只是创建了JavaThread对象。
  if (!main_thread->set_as_starting_thread()) {
    vm_shutdown_during_initialization(
      "Failed necessary internal allocation. Out of swap space");
    delete main_thread;
    *canTryAgain = false; // don't let caller call JNI_CreateJavaVM again
    return JNI_ENOMEM;
  }

  // Enable guard page *after* os::create_main_thread(), otherwise it would
  // crash Linux VM, see notes in os_linux.cpp.
  // 初始化主线程栈
  main_thread->create_stack_guard_pages();

  // Initialize Java-Level synchronization subsystem
  // 初始化对象监视器
  ObjectMonitor::Initialize() ;

  // Second phase of bootstrapping, VM is about entering multi-thread mode
  MemTracker::bootstrap_multi_thread();

  // Initialize global modules
  /*
   * 初始化全局模块
   */
  jint status = init_globals();
  if (status != JNI_OK) {
    delete main_thread;
    *canTryAgain = false; // don't let caller call JNI_CreateJavaVM again
    return status;
  }

  // Should be done after the heap is fully created
  // 创建全局变量
  main_thread->cache_global_variables();

  HandleMark hm;

  { MutexLocker mu(Threads_lock);
    Threads::add(main_thread);
  }

  // Any JVMTI raw monitors entered in onload will transition into
  // real raw monitor. VM is setup enough here for raw monitor enter.
  JvmtiExport::transition_pending_onload_raw_monitors();

  // Fully start NMT
  MemTracker::start();

  // Create the VMThread
  { TraceTime timer("Start VMThread", TraceStartupTime);
    /*
     * 创建VMThread,该线程接受并执行指定的VMOperation
     * create函数会初始化_vm_thread和_vm_queue属性。
     */
    VMThread::create();
    // 获取创建的VMThread实例
    Thread* vmthread = VMThread::vm_thread();
    // 创建os层面的线程
    if (!os::create_thread(vmthread, os::vm_thread))
      vm_exit_during_initialization("Cannot create VM thread. Out of system resources.");

    // Wait for the VM thread to become ready, and VMThread::run to initialize
    // Monitors can have spurious returns, must always check another state flag
    {
      MutexLocker ml(Notify_lock);
      // 启动vmthread
      os::start_thread(vmthread);
      // 当前线程等待VMThread线程就绪后才会执行其他逻辑
      while (vmthread->active_handles() == NULL) {
        Notify_lock->wait();
      }
    }
  }

  assert (Universe::is_fully_initialized(), "not initialized");
  if (VerifyDuringStartup) {
    // Make sure we're starting with a clean slate.
    VM_Verify verify_op;
    VMThread::execute(&verify_op);
  }

  EXCEPTION_MARK;

  // At this point, the Universe is initialized, but we have not executed
  // any byte code.  Now is a good time (the only time) to dump out the
  // internal state of the JVM for sharing.
  if (DumpSharedSpaces) {
    MetaspaceShared::preload_and_dump(CHECK_0);
    ShouldNotReachHere();
  }

  // Always call even when there are not JVMTI environments yet, since environments
  // may be attached late and JVMTI must track phases of VM execution
  JvmtiExport::enter_start_phase();

  // Notify JVMTI agents that VM has started (JNI is up) - nop if no agents.
  JvmtiExport::post_vm_start();

  {
    TraceTime timer("Initialize java.lang classes", TraceStartupTime);

    if (EagerXrunInit && Arguments::init_libraries_at_startup()) {
      create_vm_init_libraries();
    }

    initialize_class(vmSymbols::java_lang_String(), CHECK_0);

    // Initialize java_lang.System (needed before creating the thread)
    // 加载一些运行主线程需要的类
    initialize_class(vmSymbols::java_lang_System(), CHECK_0);
    initialize_class(vmSymbols::java_lang_ThreadGroup(), CHECK_0);
    // 创建并设置初始线程组
    Handle thread_group = create_initial_thread_group(CHECK_0);
    Universe::set_main_thread_group(thread_group());
    initialize_class(vmSymbols::java_lang_Thread(), CHECK_0);
    // 创建并设置Java层面的Thread类对象
    oop thread_object = create_initial_thread(thread_group, main_thread, CHECK_0);
    main_thread->set_threadObj(thread_object);
    // Set thread status to running since main thread has
    // been started and running.
    // 将主类对应的java.lang.Thread的状态设置为RUNNABLE
    java_lang_Thread::set_thread_status(thread_object,
                                        java_lang_Thread::RUNNABLE);

    // The VM creates & returns objects of this class. Make sure it's initialized.
    initialize_class(vmSymbols::java_lang_Class(), CHECK_0);

    // The VM preresolves methods to these classes. Make sure that they get initialized
    initialize_class(vmSymbols::java_lang_reflect_Method(), CHECK_0);
    initialize_class(vmSymbols::java_lang_ref_Finalizer(),  CHECK_0);
    call_initializeSystemClass(CHECK_0);

    // get the Java runtime name after java.lang.System is initialized
    JDK_Version::set_runtime_name(get_java_runtime_name(THREAD));
    JDK_Version::set_runtime_version(get_java_runtime_version(THREAD));

    // an instance of OutOfMemory exception has been allocated earlier
    initialize_class(vmSymbols::java_lang_OutOfMemoryError(), CHECK_0);
    initialize_class(vmSymbols::java_lang_NullPointerException(), CHECK_0);
    initialize_class(vmSymbols::java_lang_ClassCastException(), CHECK_0);
    initialize_class(vmSymbols::java_lang_ArrayStoreException(), CHECK_0);
    initialize_class(vmSymbols::java_lang_ArithmeticException(), CHECK_0);
    initialize_class(vmSymbols::java_lang_StackOverflowError(), CHECK_0);
    initialize_class(vmSymbols::java_lang_IllegalMonitorStateException(), CHECK_0);
    initialize_class(vmSymbols::java_lang_IllegalArgumentException(), CHECK_0);
  }

  // See        : bugid 4211085.
  // Background : the static initializer of java.lang.Compiler tries to read
  //              property"java.compiler" and read & write property "java.vm.info".
  //              When a security manager is installed through the command line
  //              option "-Djava.security.manager", the above properties are not
  //              readable and the static initializer for java.lang.Compiler fails
  //              resulting in a NoClassDefFoundError.  This can happen in any
  //              user code which calls methods in java.lang.Compiler.
  // Hack :       the hack is to pre-load and initialize this class, so that only
  //              system domains are on the stack when the properties are read.
  //              Currently even the AWT code has calls to methods in java.lang.Compiler.
  //              On the classic VM, java.lang.Compiler is loaded very early to load the JIT.
  // Future Fix : the best fix is to grant everyone permissions to read "java.compiler" and
  //              read and write"java.vm.info" in the default policy file. See bugid 4211383
  //              Once that is done, we should remove this hack.
  initialize_class(vmSymbols::java_lang_Compiler(), CHECK_0);

  // More hackery - the static initializer of java.lang.Compiler adds the string "nojit" to
  // the java.vm.info property if no jit gets loaded through java.lang.Compiler (the hotspot
  // compiler does not get loaded through java.lang.Compiler).  "java -version" with the
  // hotspot vm says "nojit" all the time which is confusing.  So, we reset it here.
  // This should also be taken out as soon as 4211383 gets fixed.
  reset_vm_info_property(CHECK_0);

  quicken_jni_functions();

  // Must be run after init_ft which initializes ft_enabled
  if (TRACE_INITIALIZE() != JNI_OK) {
    vm_exit_during_initialization("Failed to initialize tracing backend");
  }

  // Set flag that basic initialization has completed. Used by exceptions and various
  // debug stuff, that does not work until all basic classes have been initialized.
  set_init_completed();

#ifndef USDT2
  HS_DTRACE_PROBE(hotspot, vm__init__end);
#else /* USDT2 */
  HOTSPOT_VM_INIT_END();
#endif /* USDT2 */

  // record VM initialization completion time
#if INCLUDE_MANAGEMENT
  Management::record_vm_init_completed();
#endif // INCLUDE_MANAGEMENT

  // Compute system loader. Note that this has to occur after set_init_completed, since
  // valid exceptions may be thrown in the process.
  // Note that we do not use CHECK_0 here since we are inside an EXCEPTION_MARK and
  // set_init_completed has just been called, causing exceptions not to be shortcut
  // anymore. We call vm_exit_during_initialization directly instead.

  // 初始化_java_system_loader属性
  SystemDictionary::compute_java_system_loader(THREAD);
  if (HAS_PENDING_EXCEPTION) {
    vm_exit_during_initialization(Handle(THREAD, PENDING_EXCEPTION));
  }

#if INCLUDE_ALL_GCS
  // Support for ConcurrentMarkSweep. This should be cleaned up
  // and better encapsulated. The ugly nested if test would go away
  // once things are properly refactored. XXX YSR
  if (UseConcMarkSweepGC || UseG1GC) {
    if (UseConcMarkSweepGC) {
      ConcurrentMarkSweepThread::makeSurrogateLockerThread(THREAD);
    } else {
      ConcurrentMarkThread::makeSurrogateLockerThread(THREAD);
    }
    if (HAS_PENDING_EXCEPTION) {
      vm_exit_during_initialization(Handle(THREAD, PENDING_EXCEPTION));
    }
  }
#endif // INCLUDE_ALL_GCS

  // Always call even when there are not JVMTI environments yet, since environments
  // may be attached late and JVMTI must track phases of VM execution
  JvmtiExport::enter_live_phase();

  // Signal Dispatcher needs to be started before VMInit event is posted
  /*
   * 创建守护线程,包括Signal Dispatcher,Attach Listener,Watcher Thread等。
   */
  os::signal_init();

  // Start Attach Listener if +StartAttachListener or it can't be started lazily
  if (!DisableAttachMechanism) {
    AttachListener::vm_start();
    if (StartAttachListener || AttachListener::init_at_startup()) {
      AttachListener::init();
    }
  }

  // Launch -Xrun agents
  // Must be done in the JVMTI live phase so that for backward compatibility the JDWP
  // back-end can launch with -Xdebug -Xrunjdwp.
  if (!EagerXrunInit && Arguments::init_libraries_at_startup()) {
    create_vm_init_libraries();
  }

  // Notify JVMTI agents that VM initialization is complete - nop if no agents.
  JvmtiExport::post_vm_initialized();

  if (TRACE_START() != JNI_OK) {
    vm_exit_during_initialization("Failed to start tracing backend.");
  }

  if (CleanChunkPoolAsync) {
    Chunk::start_chunk_pool_cleaner_task();
  }

  // initialize compiler(s)
#if defined(COMPILER1) || defined(COMPILER2) || defined(SHARK)
  CompileBroker::compilation_init();
#endif

  if (EnableInvokeDynamic) {
    // Pre-initialize some JSR292 core classes to avoid deadlock during class loading.
    // It is done after compilers are initialized, because otherwise compilations of
    // signature polymorphic MH intrinsics can be missed
    // (see SystemDictionary::find_method_handle_intrinsic).
    initialize_class(vmSymbols::java_lang_invoke_MethodHandle(), CHECK_0);
    initialize_class(vmSymbols::java_lang_invoke_MemberName(), CHECK_0);
    initialize_class(vmSymbols::java_lang_invoke_MethodHandleNatives(), CHECK_0);
  }

#if INCLUDE_MANAGEMENT
  Management::initialize(THREAD);
#endif // INCLUDE_MANAGEMENT

  if (HAS_PENDING_EXCEPTION) {
    // management agent fails to start possibly due to
    // configuration problem and is responsible for printing
    // stack trace if appropriate. Simply exit VM.
    vm_exit(1);
  }

  if (Arguments::has_profile())       FlatProfiler::engage(main_thread, true);
  if (MemProfiling)                   MemProfiler::engage();
  StatSampler::engage();
  if (CheckJNICalls)                  JniPeriodicChecker::engage();

  // 初始化偏向锁
  BiasedLocking::init();

  if (JDK_Version::current().post_vm_init_hook_enabled()) {
    call_postVMInitHook(THREAD);
    // The Java side of PostVMInitHook.run must deal with all
    // exceptions and provide means of diagnosis.
    if (HAS_PENDING_EXCEPTION) {
      CLEAR_PENDING_EXCEPTION;
    }
  }

  {
      MutexLockerEx ml(PeriodicTask_lock, Mutex::_no_safepoint_check_flag);
      // Make sure the watcher thread can be started by WatcherThread::start()
      // or by dynamic enrollment.
      WatcherThread::make_startable();
      // Start up the WatcherThread if there are any periodic tasks
      // NOTE:  All PeriodicTasks should be registered by now. If they
      //   aren't, late joiners might appear to start slowly (we might
      //   take a while to process their first tick).
      if (PeriodicTask::num_tasks() > 0) {
          WatcherThread::start();
      }
  }

  // Give os specific code one last chance to start
  os::init_3();

  create_vm_timer.end();
#ifdef ASSERT
  _vm_complete = true;
#endif
  return JNI_OK;
}

重点关注调用了Threads类的create_vm方法来创建和初始化JVM,这个函数非常长,本文不分析,后面单独分析到某个模块时,再来介绍其在初始化过程中的操作。

加载主类

c
jdk/src/share/bin/java.c
static jclass
LoadMainClass(JNIEnv *env, int mode, char *name)
{
    jmethodID mid;
    jstring str;
    jobject result;
    jlong start, end;
    // 加载sun.launcher.LauncherHelper类
    jclass cls = GetLauncherHelperClass(env);
    NULL_CHECK0(cls);
    if (JLI_IsTraceLauncher()) {
        start = CounterGet();
    }
    /*
     * 获取sun.launcher.LauncherHelper类中定义的checkAndLoadMain()方法的指针
     */
    NULL_CHECK0(mid = (*env)->GetStaticMethodID(env, cls,
                "checkAndLoadMain", // 方法名称
                "(ZILjava/lang/String;)Ljava/lang/Class;")); // 方法签名

    str = NewPlatformString(env, name);
    /*
     * 调用sun.launcher.LauncherHelper类中的checkAndLoadMain()方法
     * 参考jni.cpp中的jni_invoke_static()函数
     */
    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;
}

该函数中在获取并调用sun.launcher.LauncherHelper类中的checkAndLoadMain方法。该方法中会利用系统类加载器来加载主类。

执行Java主类中的main方法

cpp
hotspot/src/share/vm/prims/jni.cpp
JNI_ENTRY(void, jni_CallStaticVoidMethod(JNIEnv *env, jclass cls, jmethodID methodID, ...))
  JNIWrapper("CallStaticVoidMethod");
#ifndef USDT2
  DTRACE_PROBE3(hotspot_jni, CallStaticVoidMethod__entry, env, cls, methodID);
#else /* USDT2 */
  HOTSPOT_JNI_CALLSTATICVOIDMETHOD_ENTRY(
                                         env, cls, (uintptr_t) methodID);
#endif /* USDT2 */
  DT_VOID_RETURN_MARK(CallStaticVoidMethod);

  va_list args;
  va_start(args, methodID);
  JavaValue jvalue(T_VOID);
  // 封装参数
  JNI_ArgumentPusherVaArg ap(methodID, args);
  // 调用方法
  jni_invoke_static(env, &jvalue, NULL, JNI_STATIC, methodID, &ap, CHECK);
  va_end(args);
JNI_END

通过JNI调用Java主类中的main方法。