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。
