深入理解Java之jvm启动流程

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 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 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。

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。

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。

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