JVM系列(一)：jvm启动过程速览

　　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: http://hg.openjdk.java.net/jdk8u/jdk8u/jdk/   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，都是耍流氓。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，以及随时可能与操作系统通信获取信息的能力（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 JAVAW
char **__initenv;
int WINAPI
WinMain(HINSTANCE inst, HINSTANCE previnst, LPSTR cmdline, int cmdshow)
{
    int margc;
    char** margv;
    const jboolean const_javaw = JNI_TRUE;
    __initenv = _environ;
#else /* JAVAW */
int
main(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，重点是我们都是用 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.
 */
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;
    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 void
SelectVersion(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).
 */
int
JLI_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.
 */
jboolean
LoadJavaVM(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 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;
    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.
 */
void
SetJavaCommandLineProp(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 classpath
static void
SetClassPath(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.
 */
void
AddOption(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.c
JVMInit(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
 */
void
ShowSplashScreen()
{
    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);
}


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) {
         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。