jvm是java的核心运行平台，自然是个非常复杂的系统。当然了，说jvm是个平台，实际上也是个泛称。准确的说，它是一个java虚拟机的统称，它并不指具体的某个虚拟机。所以，谈到java虚拟机时，往往我们通常说的都是一些规范性质的东西。

那么，如果想要研究jvm是如何工作的，就不能是泛泛而谈了。我们必须要具体到某个指定的虚拟机实现，以便说清其过程。

1. 说说openjdk

因为java实际上已经被oracle控制，而oracle本身是个商业公司，所以从某种程度上说，这里的java并不是完全开源的。我们称官方的jdk为oraclejdk. 或者叫 hotspot vm.

与此同时，社区维护了一个完全开源的版本，openjdk。这两个jdk实际上，大部分是相同的，只是维护的进度不太一样，以及版权归属不一样。

所以，如果想研究jvm的实现，那么基于openjdk来做，是比较明智的选择。

如果想了解openjdk是如何设计的，以及它有什么高级特性，以及各种最佳实践，那么买一本书是最佳选择。

如果业有余力，想去了解了解源码的，那么可以到官网查看源码。openjdk8的源码地址为: 因为是国外网站的原因，速度不会很快。所以只是在网站上查看源码，还是有点累的。另外，没有ide的帮助，估计很少有人能够坚持下去。另外的下载地址，大家可以网上搜索下，资源总是有的，国人链接速度快。多花点心思找找。

当然要说明的一点是：一个没有设计背景，没有框架概念的源码阅读，都是而流氓。那样的工作，就像是空中楼阁，并不让人踏实。

2. 谈谈C语言

C语言，一般作为我们的大学入门语言，或多或少都接触过。但要说精通，可能就是很少一部分人了。但我要说的是，只要学过C语言，对于大部分的程序阅读，基本上就不是问题了。

openjdk的实现中，其核心的一部分就是使用C语言写的，当然其他很多语言也是一样的。所以，C语言相当重要，在底层的世界里。这里只是说它重要，但并不代表它就一定最厉害，即不是写C语言的GG就比写JAVA的JJ厉害了。因为，工作不分高低，语言同样。只是各有所长罢了。重点不是在这里，在于思想。

C语言的编程几大流程：写代码（最核心）、编译、链接（最麻烦）、运行。

当然，最核心的自然是写代码。不对，最核心的是：做设计。

C语言中，以一个main()函数为入口，编写各种逻辑后，通过调用和控制main()方法，实现各种复杂逻辑。

所以，要研究一个项目，首先就是要找到其入口。然后根据目的，再进行各功能实现的通路学习。

C语言有极其灵活的语法，超级复杂的指针设计，以及各类似面向对象思想的结构体，以及随时可能操作系统获取信息的能力（各种链接）。所以，导致C语言有时确实比较难以读懂。这也是没办法的事，会很容易，精却很难。这是亘古不变的道理。是一个选择题，也是一道应用题。

一句话，会一点，就够吃瓜群众使用了。

3. openjdk的入口

上面说到，要研究一个C项目，首要就是找到其入口。那么，openjdk的入口在哪呢？

是在 share/bin/main.c 中，main()方法就是其入口。这个文件命名，够清晰了吧，明眼人一看就知道了。哈哈，不过一般地，我们还是需要通过查资料才知晓。

main.c是jvm的唯一main方法入口，其中，jdk被编译出来之后，会有许多的工作箱，如jmap,jps,jstack.... 这些工具箱的入口，实际也是这个main, 只是它们包含了不同的子模块，从而达到不同工具的目的。

main.c的内容也不多，主要它也只是一个框架，为屏蔽各系统的差异。它的存在，主要是为引入 JLI_LAUNCH() 方法，相当于定义自己的main()方法。

/* * This file contains the main entry point into the launcher code * this is the only file which will be repeatedly compiled by other * tools. The rest of the files will be linked in. */#include "defines.h"#ifdef _MSC_VER#if _MSC_VER > 1400 && _MSC_VER < 1600/* * When building for Microsoft Windows, main has a dependency on msvcr??.dll. * * When using Visual Studio 2005 or 2008, that must be recorded in * the [java,javaw].exe.manifest file. * * As of VS2010 (ver=1600), the runtimes again no longer need manifests. * * Reference: *     C:/Program Files/Microsoft SDKs/Windows/v6.1/include/crtdefs.h */#include <crtassem.h>#ifdef _M_IX86#pragma comment(linker,"/manifestdependency:\"type='win32' "            \        "name='" __LIBRARIES_ASSEMBLY_NAME_PREFIX ".CRT' "              \        "version='" _CRT_ASSEMBLY_VERSION "' "                          \        "processorArchitecture='x86' "                                  \        "publicKeyToken='" _VC_ASSEMBLY_PUBLICKEYTOKEN "'\"")#endif /* _M_IX86 *///This may not be necessary yet for the Windows 64-bit build, but it//will be when that build environment is updated.  Need to test to see//if it is harmless:#ifdef _M_AMD64#pragma comment(linker,"/manifestdependency:\"type='win32' "            \        "name='" __LIBRARIES_ASSEMBLY_NAME_PREFIX ".CRT' "              \        "version='" _CRT_ASSEMBLY_VERSION "' "                          \        "processorArchitecture='amd64' "                                \        "publicKeyToken='" _VC_ASSEMBLY_PUBLICKEYTOKEN "'\"")#endif  /* _M_AMD64 */#endif  /* _MSC_VER > 1400 && _MSC_VER < 1600 */#endif  /* _MSC_VER *//* * Entry point. */// 定义入口函数，JAVAW模式下使用 WinMain(), 否则使用 main()#ifdef JAVAWchar **__initenv;int WINAPIWinMain(HINSTANCE inst, HINSTANCE previnst, LPSTR cmdline, int cmdshow){    int margc;    char** margv;    const jboolean const_javaw = JNI_TRUE;    __initenv = _environ;#else /* JAVAW */intmain(int argc, char **argv){    int margc;    char** margv;    const jboolean const_javaw = JNI_FALSE;#endif /* JAVAW */#ifdef _WIN32    // windows下的参数获取    {        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 */    // 各种linux平台上的参数，直接取自main入参    margc = argc;    margv = argv;#endif /* WIN32 */    // 核心: 重新定义入口方法为: JLI_Launch()    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);}

因为java语言被设计成跨平台的语言，那么如何跨平台呢？因为平台差异总是存在的，如果语言本身不关注平台，那么自然是有人在背后关注了平台，从而屏蔽掉了差异。是了，这就是虚拟机存在的意义。因此，在入口方法，我们就可以看到，它一上来就关注平台差异性。这是必须的。

4. openjdk的启动流程

有了上面的入口知识，好像是明白了一些道理。但是好像还是没有达到要理解启动过程的目的。不急，且听我慢慢道来。

我们启动一个虚拟机时，一般是使用 java -classpath:xxx <other-options> xx.xx , 或者是 java -jar <other-options> xx.jar 。 具体怎么用无所谓，重点是我们都是 java这个应用程序启动的虚拟机。因此，我们便知道 java 程序，是我们启动jvm的核心开关。

4.0. jvm启动流程框架

废话不多说，java.c, 是我们要研究的重要文件。它将是一个控制启动流程的实现超人。而它的入口，就是在main()中的定义 JLI_Launch(...) ， 所以让我们一睹真容。

// 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;    // 初始化启动器    InitLauncher(javaw);    // 打印状态    DumpState();    // 跟踪调用启动    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.)     */    // 解析命令行参数，选择一jre版本    SelectVersion(argc, argv, &main_class);    CreateExecutionEnvironment(&argc, &argv,                               jrepath, sizeof(jrepath),                               jvmpath, sizeof(jvmpath),                               jvmcfg,  sizeof(jvmcfg));    if (!IsJavaArgs()) {        // 设置一些特殊的环境变量        SetJvmEnvironment(argc,argv);    }    ifn.CreateJavaVM = 0;    ifn.GetDefaultJavaVMInitArgs = 0;    if (JLI_IsTraceLauncher()) {        start = CounterGet();     // 记录启动时间    }    // 加载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;    // 解析更多参数信息    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，即加载java程序开始，应用表演时间到    return JVMInit(&ifn, threadStackSize, argc, argv, mode, what, ret);}

以上就是整个jvm虚拟机的启动过程框架了，基本上跑不掉几个点，就是解析命令行参数，设置参数到某范围内或者环境变量中。加载必要模块，传递变量存储。初始化系统。解析用户系统实现。当然一般地，就是会实现系统主循环，这个动作是由使用系统完成的，jvm只负责执行即可。

因为我们只是想了解大概，所以不以为然，只是其中任何一个点都足够研究很久很久了。抛开那些不说，捡个芝麻先。需要明白：懂得许多的道理却依然过不好这一生。只能安心做个吃瓜群众。

下面，就一些细节点，我们可以视兴趣，稍微深入了解下！

4.1. jre版本选择过程

以上框架中，几个重要的节点，我们可以再细化下实现。细节就不说，太复杂。首先，就是如何确定当前系统使用的jre版本，这很重要，它决定了应用系统是否可以运行的问题。因为有时候，系统的使用者并非开发者，一定存在正确的jre版本。没有jre的环境，所有java执行就会是一句空谈。

// java.c/* * The SelectVersion() routine ensures that an appropriate version of * the JRE is running.  The specification for the appropriate version * is obtained from either the manifest of a jar file (preferred) or * from command line options. * The routine also parses splash screen command line options and * passes on their values in private environment variables. */static voidSelectVersion(int argc, char **argv, char **main_class){    char    *arg;    char    **new_argv;    char    **new_argp;    char    *operand;    char    *version = NULL;    char    *jre = NULL;    int     jarflag = 0;    int     headlessflag = 0;    int     restrict_search = -1;               /* -1 implies not known */    manifest_info info;    char    env_entry[MAXNAMELEN + 24] = ENV_ENTRY "=";    char    *splash_file_name = NULL;    char    *splash_jar_name = NULL;    char    *env_in;    int     res;    /*     * If the version has already been selected, set *main_class     * with the value passed through the environment (if any) and     * simply return.     */    // _JAVA_VERSION_SET=    if ((env_in = getenv(ENV_ENTRY)) != NULL) {        if (*env_in != '\0')            *main_class = JLI_StringDup(env_in);        return;    }    /*     * Scan through the arguments for options relevant to multiple JRE     * support.  For reference, the command line syntax is defined as:     *     * SYNOPSIS     *      java [options] class [argument...]     *     *      java [options] -jar file.jar [argument...]     *     * As the scan is performed, make a copy of the argument list with     * the version specification options (new to 1.5) removed, so that     * a version less than 1.5 can be exec'd.     *     * Note that due to the syntax of the native Windows interface     * CreateProcess(), processing similar to the following exists in     * the Windows platform specific routine ExecJRE (in java_md.c).     * Changes here should be reproduced there.     */    new_argv = JLI_MemAlloc((argc + 1) * sizeof(char*));    new_argv[0] = argv[0];    new_argp = &new_argv[1];    argc--;    argv++;    while ((arg = *argv) != 0 && *arg == '-') {        if (JLI_StrCCmp(arg, "-version:") == 0) {            version = arg + 9;        } else if (JLI_StrCmp(arg, "-jre-restrict-search") == 0) {            restrict_search = 1;        } else if (JLI_StrCmp(arg, "-no-jre-restrict-search") == 0) {            restrict_search = 0;        } else {            if (JLI_StrCmp(arg, "-jar") == 0)                jarflag = 1;            /* deal with "unfortunate" classpath syntax */            if ((JLI_StrCmp(arg, "-classpath") == 0 || JLI_StrCmp(arg, "-cp") == 0) &&              (argc >= 2)) {                *new_argp++ = arg;                argc--;                argv++;                arg = *argv;            }            /*             * Checking for headless toolkit option in the some way as AWT does:             * "true" means true and any other value means false             */            if (JLI_StrCmp(arg, "-Djava.awt.headless=true") == 0) {                headlessflag = 1;            } else if (JLI_StrCCmp(arg, "-Djava.awt.headless=") == 0) {                headlessflag = 0;            } else if (JLI_StrCCmp(arg, "-splash:") == 0) {                splash_file_name = arg+8;            }            *new_argp++ = arg;        }        argc--;        argv++;    }    if (argc <= 0) {    /* No operand? Possibly legit with -[full]version */        operand = NULL;    } else {        argc--;        *new_argp++ = operand = *argv++;    }    while (argc-- > 0)  /* Copy over [argument...] */        *new_argp++ = *argv++;    *new_argp = NULL;    /*     * If there is a jar file, read the manifest. If the jarfile can't be     * read, the manifest can't be read from the jar file, or the manifest     * is corrupt, issue the appropriate error messages and exit.     *     * Even if there isn't a jar file, construct a manifest_info structure     * containing the command line information.  It's a convenient way to carry     * this data around.     */    if (jarflag && operand) {        if ((res = JLI_ParseManifest(operand, &info)) != 0) {            if (res == -1)                JLI_ReportErrorMessage(JAR_ERROR2, operand);            else                JLI_ReportErrorMessage(JAR_ERROR3, operand);            exit(1);        }        /*         * Command line splash screen option should have precedence         * over the manifest, so the manifest data is used only if         * splash_file_name has not been initialized above during command         * line parsing         */        if (!headlessflag && !splash_file_name && info.splashscreen_image_file_name) {            splash_file_name = info.splashscreen_image_file_name;            splash_jar_name = operand;        }    } else {        info.manifest_version = NULL;        info.main_class = NULL;        info.jre_version = NULL;        info.jre_restrict_search = 0;    }    /*     * Passing on splash screen info in environment variables     */    if (splash_file_name && !headlessflag) {        char* splash_file_entry = JLI_MemAlloc(JLI_StrLen(SPLASH_FILE_ENV_ENTRY "=")+JLI_StrLen(splash_file_name)+1);        JLI_StrCpy(splash_file_entry, SPLASH_FILE_ENV_ENTRY "=");        JLI_StrCat(splash_file_entry, splash_file_name);        putenv(splash_file_entry);    }    if (splash_jar_name && !headlessflag) {        char* splash_jar_entry = JLI_MemAlloc(JLI_StrLen(SPLASH_JAR_ENV_ENTRY "=")+JLI_StrLen(splash_jar_name)+1);        JLI_StrCpy(splash_jar_entry, SPLASH_JAR_ENV_ENTRY "=");        JLI_StrCat(splash_jar_entry, splash_jar_name);        putenv(splash_jar_entry);    }    /*     * The JRE-Version and JRE-Restrict-Search values (if any) from the     * manifest are overwritten by any specified on the command line.     */    if (version != NULL)        info.jre_version = version;    if (restrict_search != -1)        info.jre_restrict_search = restrict_search;    /*     * "Valid" returns (other than unrecoverable errors) follow.  Set     * main_class as a side-effect of this routine.     */    if (info.main_class != NULL)        *main_class = JLI_StringDup(info.main_class);    /*     * If no version selection information is found either on the command     * line or in the manifest, simply return.     */    if (info.jre_version == NULL) {        JLI_FreeManifest();        JLI_MemFree(new_argv);        return;    }    /*     * Check for correct syntax of the version specification (JSR 56).     */    if (!JLI_ValidVersionString(info.jre_version)) {        JLI_ReportErrorMessage(SPC_ERROR1, info.jre_version);        exit(1);    }    /*     * Find the appropriate JVM on the system. Just to be as forgiving as     * possible, if the standard algorithms don't locate an appropriate     * jre, check to see if the one running will satisfy the requirements.     * This can happen on systems which haven't been set-up for multiple     * JRE support.     */    jre = LocateJRE(&info);    JLI_TraceLauncher("JRE-Version = %s, JRE-Restrict-Search = %s Selected = %s\n",        (info.jre_version?info.jre_version:"null"),        (info.jre_restrict_search?"true":"false"), (jre?jre:"null"));    if (jre == NULL) {        if (JLI_AcceptableRelease(GetFullVersion(), info.jre_version)) {            JLI_FreeManifest();            JLI_MemFree(new_argv);            return;        } else {            JLI_ReportErrorMessage(CFG_ERROR4, info.jre_version);            exit(1);        }    }    /*     * If I'm not the chosen one, exec the chosen one.  Returning from     * ExecJRE indicates that I am indeed the chosen one.     *     * The private environment variable _JAVA_VERSION_SET is used to     * prevent the chosen one from re-reading the manifest file and     * using the values found within to override the (potential) command     * line flags stripped from argv (because the target may not     * understand them).  Passing the MainClass value is an optimization     * to avoid locating, expanding and parsing the manifest extra     * times.     */    if (info.main_class != NULL) {        if (JLI_StrLen(info.main_class) <= MAXNAMELEN) {            (void)JLI_StrCat(env_entry, info.main_class);        } else {            JLI_ReportErrorMessage(CLS_ERROR5, MAXNAMELEN);            exit(1);        }    }    (void)putenv(env_entry);    ExecJRE(jre, new_argv);    JLI_FreeManifest();    JLI_MemFree(new_argv);    return;}

逻辑也不复杂，大概就是，解析参数，读取manifest文件，jre版本校验，加载jre以便确认是否存在，最后将相关环境变量放置好。

manifest解析过程：

// share/bin/parse_manifest.c/* * Read the manifest from the specified jar file and fill in the manifest_info * structure with the information found within. * * Error returns are as follows: *    0 Success *   -1 Unable to open jarfile *   -2 Error accessing the manifest from within the jarfile (most likely *      a manifest is not present, or this isn't a valid zip/jar file). */intJLI_ParseManifest(char *jarfile, manifest_info *info){    int     fd;    zentry  entry;    char    *lp;    char    *name;    char    *value;    int     rc;    char    *splashscreen_name = NULL;    if ((fd = open(jarfile, O_RDONLY#ifdef O_LARGEFILE        | O_LARGEFILE /* large file mode */#endif#ifdef O_BINARY        | O_BINARY /* use binary mode on windows */#endif        )) == -1) {        return (-1);    }    info->manifest_version = NULL;    info->main_class = NULL;    info->jre_version = NULL;    info->jre_restrict_search = 0;    info->splashscreen_image_file_name = NULL;    // *manifest_name = "META-INF/MANIFEST.MF";    if (rc = find_file(fd, &entry, manifest_name) != 0) {        close(fd);        return (-2);    }    manifest = inflate_file(fd, &entry, NULL);    if (manifest == NULL) {        close(fd);        return (-2);    }    lp = manifest;    while ((rc = parse_nv_pair(&lp, &name, &value)) > 0) {        if (JLI_StrCaseCmp(name, "Manifest-Version") == 0)            info->manifest_version = value;        else if (JLI_StrCaseCmp(name, "Main-Class") == 0)            info->main_class = value;        else if (JLI_StrCaseCmp(name, "JRE-Version") == 0)            info->jre_version = value;        else if (JLI_StrCaseCmp(name, "JRE-Restrict-Search") == 0) {            if (JLI_StrCaseCmp(value, "true") == 0)                info->jre_restrict_search = 1;        } else if (JLI_StrCaseCmp(name, "Splashscreen-Image") == 0) {            info->splashscreen_image_file_name = value;        }    }    close(fd);    if (rc == 0)        return (0);    else        return (-2);}

manifest中只解析 几个特定的值，写多了也没用。

4.2. 加载VM模块

加载VM是非常重要的一个工作。它是一个平台相关的实现，我们看下 windows版本的实现吧。

// share/windows/bin/java_md.c/* * Load a jvm from "jvmpath" and initialize the invocation functions. */jbooleanLoadJavaVM(const char *jvmpath, InvocationFunctions *ifn){    HINSTANCE handle;    JLI_TraceLauncher("JVM path is %s\n", jvmpath);    /*     * The Microsoft C Runtime Library needs to be loaded first.  A copy is     * assumed to be present in the "JRE path" directory.  If it is not found     * there (or "JRE path" fails to resolve), skip the explicit load and let     * nature take its course, which is likely to be a failure to execute.     *     */    LoadMSVCRT();    // windows 中是通过路径加载dll文件实现    /* Load the Java VM DLL */    if ((handle = LoadLibrary(jvmpath)) == 0) {        JLI_ReportErrorMessage(DLL_ERROR4, (char *)jvmpath);        return JNI_FALSE;    }    /* Now get the function addresses */    // 获取虚拟机操作内存地址    ifn->CreateJavaVM =        (void *)GetProcAddress(handle, "JNI_CreateJavaVM");    ifn->GetDefaultJavaVMInitArgs =        (void *)GetProcAddress(handle, "JNI_GetDefaultJavaVMInitArgs");    if (ifn->CreateJavaVM == 0 || ifn->GetDefaultJavaVMInitArgs == 0) {        JLI_ReportErrorMessage(JNI_ERROR1, (char *)jvmpath);        return JNI_FALSE;    }    return JNI_TRUE;}

可见，最重要的工作是被封装到 JRE 中的，应用层面只是调用JRE的方法即可。在windows中通过加载 msvcrt 模块完成工作，然后抽取vm的两个方法签名到 ifn 中，以便后续实用。

4.3. 解析参数信息

通过参数解析，我们就可以如何设置参数了。更深层次的理解。

// 实际就是语法规范/* * Parses command line arguments.  Returns JNI_FALSE if launcher * should exit without starting vm, returns JNI_TRUE if vm needs * to be started to process given options.  *pret (the launcher * process return value) is set to 0 for a normal exit. */static jbooleanParseArguments(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;    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;}/* * inject the -Dsun.java.command pseudo property into the args structure * this pseudo property is used in the HotSpot VM to expose the * Java class name and arguments to the main method to the VM. The * HotSpot VM uses this pseudo property to store the Java class name * (or jar file name) and the arguments to the class's main method * to the instrumentation memory region. The sun.java.command pseudo * property is not exported by HotSpot to the Java layer. */voidSetJavaCommandLineProp(char *what, int argc, char **argv){    int i = 0;    size_t len = 0;    char* javaCommand = NULL;    char* dashDstr = "-Dsun.java.command=";    if (what == NULL) {        /* unexpected, one of these should be set. just return without         * setting the property         */        return;    }    /* determine the amount of memory to allocate assuming     * the individual components will be space separated     */    len = JLI_StrLen(what);    for (i = 0; i < argc; i++) {        len += JLI_StrLen(argv[i]) + 1;    }    /* allocate the memory */    javaCommand = (char*) JLI_MemAlloc(len + JLI_StrLen(dashDstr) + 1);    /* build the -D string */    *javaCommand = '\0';    JLI_StrCat(javaCommand, dashDstr);    JLI_StrCat(javaCommand, what);    for (i = 0; i < argc; i++) {        /* the components of the string are space separated. In         * the case of embedded white space, the relationship of         * the white space separated components to their true         * positional arguments will be ambiguous. This issue may         * be addressed in a future release.         */        JLI_StrCat(javaCommand, " ");        JLI_StrCat(javaCommand, argv[i]);    }    AddOption(javaCommand, NULL);}// 设置 classpathstatic voidSetClassPath(const char *s){    char *def;    const char *orig = s;    static const char format[] = "-Djava.class.path=%s";    /*     * usually we should not get a null pointer, but there are cases where     * we might just get one, in which case we simply ignore it, and let the     * caller deal with it     */    if (s == NULL)        return;    s = JLI_WildcardExpandClasspath(s);    if (sizeof(format) - 2 + JLI_StrLen(s) < JLI_StrLen(s))        // s is corrupted after wildcard expansion        return;    def = JLI_MemAlloc(sizeof(format)                       - 2 /* strlen("%s") */                       + JLI_StrLen(s));    sprintf(def, format, s);    AddOption(def, NULL);    if (s != orig)        JLI_MemFree((char *) s);}

-Xxxxx, --xxx 格式配置，如 -Xms1024G, --noclassgc ... 然后解析出来。 最后通过 AddOption() 存储起来。在AddOption() 通过一个全局的 options 选项，保存各参数配置。

/* * Adds a new VM option with the given given name and value. */voidAddOption(char *str, void *info){    /*     * Expand options array if needed to accommodate at least one more     * VM option.  以2倍的形式进行扩容。     */    if (numOptions >= maxOptions) {        if (options == 0) {            maxOptions = 4;            options = JLI_MemAlloc(maxOptions * sizeof(JavaVMOption));        } else {            JavaVMOption *tmp;            maxOptions *= 2;            tmp = JLI_MemAlloc(maxOptions * sizeof(JavaVMOption));            memcpy(tmp, options, numOptions * sizeof(JavaVMOption));            JLI_MemFree(options);            options = tmp;        }    }    options[numOptions].optionString = str;    options[numOptions++].extraInfo = info;    if (JLI_StrCCmp(str, "-Xss") == 0) {        jlong tmp;        if (parse_size(str + 4, &tmp)) {            threadStackSize = tmp;        }    }    if (JLI_StrCCmp(str, "-Xmx") == 0) {        jlong tmp;        if (parse_size(str + 4, &tmp)) {            maxHeapSize = tmp;        }    }    if (JLI_StrCCmp(str, "-Xms") == 0) {        jlong tmp;        if (parse_size(str + 4, &tmp)) {           initialHeapSize = tmp;        }    }}

从以上，我们也可以得知一个事情，即当设置了多次的堆大小 -Xmx, -Xms 时，会以最后一个设置为准。

4.4. jvm初始化

好像我们一直讨论的都是这个，但是实际上里面还有一个真正的jvm的初始化过程。这里方才会接入真正的java程序，也才大家所关心的地方。

// java.cJVMInit(InvocationFunctions* ifn, jlong threadStackSize,        int argc, char **argv,        int mode, char *what, int ret){    ShowSplashScreen();    return ContinueInNewThread(ifn, threadStackSize, argc, argv, mode, what, ret);}/* * Displays the splash screen according to the jar file name * and image file names stored in environment variables */voidShowSplashScreen(){    const char *jar_name = getenv(SPLASH_JAR_ENV_ENTRY);    const char *file_name = getenv(SPLASH_FILE_ENV_ENTRY);    int data_size;    void *image_data = NULL;    float scale_factor = 1;    char *scaled_splash_name = NULL;    if (file_name == NULL){        return;    }    scaled_splash_name = DoSplashGetScaledImageName(                        jar_name, file_name, &scale_factor);    if (jar_name) {        if (scaled_splash_name) {            image_data = JLI_JarUnpackFile(                    jar_name, scaled_splash_name, &data_size);        }        if (!image_data) {            scale_factor = 1;            image_data = JLI_JarUnpackFile(                            jar_name, file_name, &data_size);        }        if (image_data) {            DoSplashInit();            DoSplashSetScaleFactor(scale_factor);            DoSplashLoadMemory(image_data, data_size);            JLI_MemFree(image_data);        }    } else {        DoSplashInit();        if (scaled_splash_name) {            DoSplashSetScaleFactor(scale_factor);            DoSplashLoadFile(scaled_splash_name);        } else {            DoSplashLoadFile(file_name);        }    }    if (scaled_splash_name) {        JLI_MemFree(scaled_splash_name);    }    DoSplashSetFileJarName(file_name, jar_name);    /*     * Done with all command line processing and potential re-execs so     * clean up the environment.     */    (void)UnsetEnv(ENV_ENTRY);    (void)UnsetEnv(SPLASH_FILE_ENV_ENTRY);    (void)UnsetEnv(SPLASH_JAR_ENV_ENTRY);    JLI_MemFree(splash_jar_entry);    JLI_MemFree(splash_file_entry);}intContinueInNewThread(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;      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;    }}

看起来，jvm是通过一个新线程去运行应用系统的。在将执行控制权交由java代码后，它的主要作用，就是不停地接收命令，执行命令。从而变成一个真正的执行机器。

火车已开，后续更精彩。

看完三件事❤️

如果你觉得这篇内容对你还蛮有帮助，我想邀请你帮我三个小忙：

点赞，转发，有你们的 『点赞和评论』，才是我创造的动力。关注头条号 『 JAVA后端架构 』，不定期分享原创知识。同时可以期待后续文章ing关注作者后台私信【888】有惊喜相送

作者：等你归去来

出处：