深入理解Java之jvm启动流程

2021-02-08 17:55:32 来源:易采站长站 作者:

深入理解Java之jvm启动流程,语言,入口,参数,都是,方法

深入理解Java之jvm启动流程

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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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