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。
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/* * this file contains the main entry point into the launcher code * this is the only file which will be repeatedly compiled by other * tools. the rest of the files will be linked in. */#include "defines.h"#ifdef _msc_ver#if _msc_ver > 1400 && _msc_ver < 1600/* * when building for microsoft windows, main has a dependency on msvcr??.dll. * * when using visual studio 2005 or 2008, that must be recorded in * the [java,javaw].exe.manifest file. * * as of vs2010 (ver=1600), the runtimes again no longer need manifests. * * reference: * c:/program files/microsoft sdks/windows/v6.1/include/crtdefs.h */#include <crtassem.h>#ifdef _m_ix86#pragma comment(linker,"/manifestdependency:\"type='win32' " \ "name='" __libraries_assembly_name_prefix ".crt' " \ "version='" _crt_assembly_version "' " \ "processorarchitecture='x86' " \ "publickeytoken='" _vc_assembly_publickeytoken "'\"")#endif /* _m_ix86 *///this may not be necessary yet for the windows 64-bit build, but it//will be when that build environment is updated. need to test to see//if it is harmless:#ifdef _m_amd64#pragma comment(linker,"/manifestdependency:\"type='win32' " \ "name='" __libraries_assembly_name_prefix ".crt' " \ "version='" _crt_assembly_version "' " \ "processorarchitecture='amd64' " \ "publickeytoken='" _vc_assembly_publickeytoken "'\"")#endif /* _m_amd64 */#endif /* _msc_ver > 1400 && _msc_ver < 1600 */#endif /* _msc_ver *//* * entry point. */// 定义入口函数，javaw模式下使用 winmain(), 否则使用 main()#ifdef javawchar **__initenv;int winapiwinmain(hinstance inst, hinstance previnst, lpstr cmdline, int cmdshow){ int margc; char** margv; const jboolean const_javaw = jni_true; __initenv = _environ;#else /* javaw */intmain(int argc, char **argv){ int margc; char** margv; const jboolean const_javaw = jni_false;#endif /* javaw */#ifdef _win32 // windows下的参数获取 { int i = 0; if (getenv(jldebug_env_entry) != null) { printf("windows original main args:\n"); for (i = 0 ; i < __argc ; i++) { printf("wwwd_args[%d] = %s\n", i, __argv[i]); } } } jli_cmdtoargs(getcommandline()); margc = jli_getstdargc(); // add one more to mark the end margv = (char **)jli_memalloc((margc + 1) * (sizeof(char *))); { int i = 0; stdarg *stdargs = jli_getstdargs(); for (i = 0 ; i < margc ; i++) { margv[i] = stdargs[i].arg; } margv[i] = null; }#else /* *nixes */ // 各种linux平台上的参数，直接取自main入参 margc = argc; margv = argv;#endif /* win32 */ // 核心: 重新定义入口方法为: jli_launch() return jli_launch(margc, margv, sizeof(const_jargs) / sizeof(char *), const_jargs, sizeof(const_appclasspath) / sizeof(char *), const_appclasspath, full_version, dot_version, (const_progname != null) ? const_progname : *margv, (const_launcher != null) ? const_launcher : *margv, (const_jargs != null) ? jni_true : jni_false, const_cpwildcard, const_javaw, const_ergo_class);} |
因为java语言被设计成跨平台的语言，那么如何跨平台呢？因为平台差异总是存在的，如果语言本身不关注平台，那么自然是有人在背后关注了平台，从而屏蔽掉了差异。是了，这就是虚拟机存在的意义。因此，在入口方法，我们就可以看到，它一上来就关注平台差异性。这是必须的。
4. openjdk的启动流程
有了上面的入口知识，好像是明白了一些道理。但是好像还是没有达到要理解启动过程的目的。不急，且听我慢慢道来。
我们启动一个虚拟机时，一般是使用 java -classpath:xxx <other-options> xx.xx , 或者是 java -jar <other-options> xx.jar 。 具体怎么用无所谓，重点是我们都是 java这个应用程序启动的虚拟机。因此，我们便知道 java 程序，是我们启动jvm的核心开关。
4.0. jvm启动流程框架
废话不多说，java.c, 是我们要研究的重要文件。它将是一个控制启动流程的实现超人。而它的入口，就是在main()中的定义 jli_launch(...) ， 所以让我们一睹真容。
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// share/bin/java.c/* * entry point. */intjli_launch(int argc, char ** argv, /* main argc, argc */ int jargc, const char** jargv, /* java args */ int appclassc, const char** appclassv, /* app classpath */ const char* fullversion, /* full version defined */ const char* dotversion, /* dot version defined */ const char* pname, /* program name */ const char* lname, /* launcher name */ jboolean javaargs, /* java_args */ jboolean cpwildcard, /* classpath wildcard*/ jboolean javaw, /* windows-only javaw */ jint ergo /* ergonomics class policy */){ int mode = lm_unknown; char *what = null; char *cpath = 0; char *main_class = null; int ret; invocationfunctions ifn; jlong start, end; char jvmpath[maxpathlen]; char jrepath[maxpathlen]; char jvmcfg[maxpathlen]; _fversion = fullversion; _dversion = dotversion; _launcher_name = lname; _program_name = pname; _is_java_args = javaargs; _wc_enabled = cpwildcard; _ergo_policy = ergo; // 初始化启动器 initlauncher(javaw); // 打印状态 dumpstate(); // 跟踪调用启动 if (jli_istracelauncher()) { int i; printf("command line args:\n"); for (i = 0; i < argc ; i++) { printf("argv[%d] = %s\n", i, argv[i]); } addoption("-dsun.java.launcher.diag=true", null); } /* * make sure the specified version of the jre is running. * * there are three things to note about the selectversion() routine: * 1) if the version running isn't correct, this routine doesn't * return (either the correct version has been exec'd or an error * was issued). * 2) argc and argv in this scope are *not* altered by this routine. * it is the responsibility of subsequent code to ignore the * arguments handled by this routine. * 3) as a side-effect, the variable "main_class" is guaranteed to * be set (if it should ever be set). this isn't exactly the * poster child for structured programming, but it is a small * price to pay for not processing a jar file operand twice. * (note: this side effect has been disabled. see comment on * bugid 5030265 below.) */ // 解析命令行参数，选择一jre版本 selectversion(argc, argv, &main_class); createexecutionenvironment(&argc, &argv, jrepath, sizeof(jrepath), jvmpath, sizeof(jvmpath), jvmcfg, sizeof(jvmcfg)); if (!isjavaargs()) { // 设置一些特殊的环境变量 setjvmenvironment(argc,argv); } ifn.createjavavm = 0; ifn.getdefaultjavavminitargs = 0; if (jli_istracelauncher()) { start = counterget(); // 记录启动时间 } // 加载vm, 重中之重 if (!loadjavavm(jvmpath, &ifn)) { return(6); } if (jli_istracelauncher()) { end = counterget(); } jli_tracelauncher("%ld micro seconds to loadjavavm\n", (long)(jint)counter2micros(end-start)); ++argv; --argc; // 解析更多参数信息 if (isjavaargs()) { /* preprocess wrapper arguments */ translateapplicationargs(jargc, jargv, &argc, &argv); if (!addapplicationoptions(appclassc, appclassv)) { return(1); } } else { /* set default classpath */ cpath = getenv("classpath"); if (cpath == null) { cpath = "."; } setclasspath(cpath); } /* parse command line options; if the return value of * parsearguments is false, the program should exit. */ // 解析参数 if (!parsearguments(&argc, &argv, &mode, &what, &ret, jrepath)) { return(ret); } /* override class path if -jar flag was specified */ if (mode == lm_jar) { setclasspath(what); /* override class path */ } /* set the -dsun.java.command pseudo property */ setjavacommandlineprop(what, argc, argv); /* set the -dsun.java.launcher pseudo property */ setjavalauncherprop(); /* set the -dsun.java.launcher.* platform properties */ setjavalauncherplatformprops(); // 初始化jvm，即加载java程序开始，应用表演时间到 return jvminit(&ifn, threadstacksize, argc, argv, mode, what, ret);} |
以上就是整个jvm虚拟机的启动过程框架了，基本上跑不掉几个点，就是解析命令行参数，设置参数到某范围内或者环境变量中。加载必要模块，传递变量存储。初始化系统。解析用户系统实现。当然一般地，就是会实现系统主循环，这个动作是由使用系统完成的，jvm只负责执行即可。
因为我们只是想了解大概，所以不以为然，只是其中任何一个点都足够研究很久很久了。抛开那些不说，捡个芝麻先。需要明白：懂得许多的道理却依然过不好这一生。只能安心做个吃瓜群众。
下面，就一些细节点，我们可以视兴趣，稍微深入了解下！
4.1. jre版本选择过程
以上框架中，几个重要的节点，我们可以再细化下实现。细节就不说，太复杂。首先，就是如何确定当前系统使用的jre版本，这很重要，它决定了应用系统是否可以运行的问题。因为有时候，系统的使用者并非开发者，一定存在正确的jre版本。没有jre的环境，所有java执行就会是一句空谈。
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// java.c/* * the selectversion() routine ensures that an appropriate version of * the jre is running. the specification for the appropriate version * is obtained from either the manifest of a jar file (preferred) or * from command line options. * the routine also parses splash screen command line options and * passes on their values in private environment variables. */static voidselectversion(int argc, char **argv, char **main_class){ char *arg; char **new_argv; char **new_argp; char *operand; char *version = null; char *jre = null; int jarflag = 0; int headlessflag = 0; int restrict_search = -1; /* -1 implies not known */ manifest_info info; char env_entry[maxnamelen + 24] = env_entry "="; char *splash_file_name = null; char *splash_jar_name = null; char *env_in; int res; /* * if the version has already been selected, set *main_class * with the value passed through the environment (if any) and * simply return. */ // _java_version_set= if ((env_in = getenv(env_entry)) != null) { if (*env_in != '\0') *main_class = jli_stringdup(env_in); return; } /* * scan through the arguments for options relevant to multiple jre * support. for reference, the command line syntax is defined as: * * synopsis * java [options] class [argument...] * * java [options] -jar file.jar [argument...] * * as the scan is performed, make a copy of the argument list with * the version specification options (new to 1.5) removed, so that * a version less than 1.5 can be exec'd. * * note that due to the syntax of the native windows interface * createprocess(), processing similar to the following exists in * the windows platform specific routine execjre (in java_md.c). * changes here should be reproduced there. */ new_argv = jli_memalloc((argc + 1) * sizeof(char*)); new_argv[0] = argv[0]; new_argp = &new_argv[1]; argc--; argv++; while ((arg = *argv) != 0 && *arg == '-') { if (jli_strccmp(arg, "-version:") == 0) { version = arg + 9; } else if (jli_strcmp(arg, "-jre-restrict-search") == 0) { restrict_search = 1; } else if (jli_strcmp(arg, "-no-jre-restrict-search") == 0) { restrict_search = 0; } else { if (jli_strcmp(arg, "-jar") == 0) jarflag = 1; /* deal with "unfortunate" classpath syntax */ if ((jli_strcmp(arg, "-classpath") == 0 || jli_strcmp(arg, "-cp") == 0) && (argc >= 2)) { *new_argp++ = arg; argc--; argv++; arg = *argv; } /* * checking for headless toolkit option in the some way as awt does: * "true" means true and any other value means false */ if (jli_strcmp(arg, "-djava.awt.headless=true") == 0) { headlessflag = 1; } else if (jli_strccmp(arg, "-djava.awt.headless=") == 0) { headlessflag = 0; } else if (jli_strccmp(arg, "-splash:") == 0) { splash_file_name = arg+8; } *new_argp++ = arg; } argc--; argv++; } if (argc <= 0) { /* no operand? possibly legit with -[full]version */ operand = null; } else { argc--; *new_argp++ = operand = *argv++; } while (argc-- > 0) /* copy over [argument...] */ *new_argp++ = *argv++; *new_argp = null; /* * if there is a jar file, read the manifest. if the jarfile can't be * read, the manifest can't be read from the jar file, or the manifest * is corrupt, issue the appropriate error messages and exit. * * even if there isn't a jar file, construct a manifest_info structure * containing the command line information. it's a convenient way to carry * this data around. */ if (jarflag && operand) { if ((res = jli_parsemanifest(operand, &info)) != 0) { if (res == -1) jli_reporterrormessage(jar_error2, operand); else jli_reporterrormessage(jar_error3, operand); exit(1); } /* * command line splash screen option should have precedence * over the manifest, so the manifest data is used only if * splash_file_name has not been initialized above during command * line parsing */ if (!headlessflag && !splash_file_name && info.splashscreen_image_file_name) { splash_file_name = info.splashscreen_image_file_name; splash_jar_name = operand; } } else { info.manifest_version = null; info.main_class = null; info.jre_version = null; info.jre_restrict_search = 0; } /* * passing on splash screen info in environment variables */ if (splash_file_name && !headlessflag) { char* splash_file_entry = jli_memalloc(jli_strlen(splash_file_env_entry "=")+jli_strlen(splash_file_name)+1); jli_strcpy(splash_file_entry, splash_file_env_entry "="); jli_strcat(splash_file_entry, splash_file_name); putenv(splash_file_entry); } if (splash_jar_name && !headlessflag) { char* splash_jar_entry = jli_memalloc(jli_strlen(splash_jar_env_entry "=")+jli_strlen(splash_jar_name)+1); jli_strcpy(splash_jar_entry, splash_jar_env_entry "="); jli_strcat(splash_jar_entry, splash_jar_name); putenv(splash_jar_entry); } /* * the jre-version and jre-restrict-search values (if any) from the * manifest are overwritten by any specified on the command line. */ if (version != null) info.jre_version = version; if (restrict_search != -1) info.jre_restrict_search = restrict_search; /* * "valid" returns (other than unrecoverable errors) follow. set * main_class as a side-effect of this routine. */ if (info.main_class != null) *main_class = jli_stringdup(info.main_class); /* * if no version selection information is found either on the command * line or in the manifest, simply return. */ if (info.jre_version == null) { jli_freemanifest(); jli_memfree(new_argv); return; } /* * check for correct syntax of the version specification (jsr 56). */ if (!jli_validversionstring(info.jre_version)) { jli_reporterrormessage(spc_error1, info.jre_version); exit(1); } /* * find the appropriate jvm on the system. just to be as forgiving as * possible, if the standard algorithms don't locate an appropriate * jre, check to see if the one running will satisfy the requirements. * this can happen on systems which haven't been set-up for multiple * jre support. */ jre = locatejre(&info); jli_tracelauncher("jre-version = %s, jre-restrict-search = %s selected = %s\n", (info.jre_version?info.jre_version:"null"), (info.jre_restrict_search?"true":"false"), (jre?jre:"null")); if (jre == null) { if (jli_acceptablerelease(getfullversion(), info.jre_version)) { jli_freemanifest(); jli_memfree(new_argv); return; } else { jli_reporterrormessage(cfg_error4, info.jre_version); exit(1); } } /* * if i'm not the chosen one, exec the chosen one. returning from * execjre indicates that i am indeed the chosen one. * * the private environment variable _java_version_set is used to * prevent the chosen one from re-reading the manifest file and * using the values found within to override the (potential) command * line flags stripped from argv (because the target may not * understand them). passing the mainclass value is an optimization * to avoid locating, expanding and parsing the manifest extra * times. */ if (info.main_class != null) { if (jli_strlen(info.main_class) <= maxnamelen) { (void)jli_strcat(env_entry, info.main_class); } else { jli_reporterrormessage(cls_error5, maxnamelen); exit(1); } } (void)putenv(env_entry); execjre(jre, new_argv); jli_freemanifest(); jli_memfree(new_argv); return;} |
逻辑也不复杂，大概就是，解析参数，读取manifest文件，jre版本校验，加载jre以便确认是否存在，最后将相关环境变量放置好。
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。
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// share/windows/bin/java_md.c/* * load a jvm from "jvmpath" and initialize the invocation functions. */jbooleanloadjavavm(const char *jvmpath, invocationfunctions *ifn){ hinstance handle; jli_tracelauncher("jvm path is %s\n", jvmpath); /* * the microsoft c runtime library needs to be loaded first. a copy is * assumed to be present in the "jre path" directory. if it is not found * there (or "jre path" fails to resolve), skip the explicit load and let * nature take its course, which is likely to be a failure to execute. * */ loadmsvcrt(); // windows 中是通过路径加载dll文件实现 /* load the java vm dll */ if ((handle = loadlibrary(jvmpath)) == 0) { jli_reporterrormessage(dll_error4, (char *)jvmpath); return jni_false; } /* now get the function addresses */ // 获取虚拟机操作内存地址 ifn->createjavavm = (void *)getprocaddress(handle, "jni_createjavavm"); ifn->getdefaultjavavminitargs = (void *)getprocaddress(handle, "jni_getdefaultjavavminitargs"); if (ifn->createjavavm == 0 || ifn->getdefaultjavavminitargs == 0) { jli_reporterrormessage(jni_error1, (char *)jvmpath); return jni_false; } return jni_true;} |
可见，最重要的工作是被封装到 jre 中的，应用层面只是调用jre的方法即可。在windows中通过加载msvcrt模块完成工作，然后抽取vm的两个方法签名到ifn中，以便后续实用。
4.3. 解析参数信息
通过参数解析，我们就可以如何设置参数了。更深层次的理解。
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// 实际就是语法规范/* * parses command line arguments. returns jni_false if launcher * should exit without starting vm, returns jni_true if vm needs * to be started to process given options. *pret (the launcher * process return value) is set to 0 for a normal exit. */static jbooleanparsearguments(int *pargc, char ***pargv, int *pmode, char **pwhat, int *pret, const char *jrepath){ int argc = *pargc; char **argv = *pargv; int mode = lm_unknown; char *arg; *pret = 0; while ((arg = *argv) != 0 && *arg == '-') { argv++; --argc; if (jli_strcmp(arg, "-classpath") == 0 || jli_strcmp(arg, "-cp") == 0) { arg_check (argc, arg_error1, arg); setclasspath(*argv); mode = lm_class; argv++; --argc; } else if (jli_strcmp(arg, "-jar") == 0) { arg_check (argc, arg_error2, arg); mode = lm_jar; } else if (jli_strcmp(arg, "-help") == 0 || jli_strcmp(arg, "-h") == 0 || jli_strcmp(arg, "-?") == 0) { printusage = jni_true; return jni_true; } else if (jli_strcmp(arg, "-version") == 0) { printversion = jni_true; return jni_true; } else if (jli_strcmp(arg, "-showversion") == 0) { showversion = jni_true; } else if (jli_strcmp(arg, "-x") == 0) { printxusage = jni_true; return jni_true;/* * the following case checks for -xshowsettings or -xshowsetting:subopt. * in the latter case, any subopt value not recognized will default to "all" */ } else if (jli_strcmp(arg, "-xshowsettings") == 0 || jli_strccmp(arg, "-xshowsettings:") == 0) { showsettings = arg; } else if (jli_strcmp(arg, "-xdiag") == 0) { addoption("-dsun.java.launcher.diag=true", null);/* * the following case provide backward compatibility with old-style * command line options. */ } else if (jli_strcmp(arg, "-fullversion") == 0) { jli_reportmessage("%s full version \"%s\"", _launcher_name, getfullversion()); return jni_false; } else if (jli_strcmp(arg, "-verbosegc") == 0) { addoption("-verbose:gc", null); } else if (jli_strcmp(arg, "-t") == 0) { addoption("-xt", null); } else if (jli_strcmp(arg, "-tm") == 0) { addoption("-xtm", null); } else if (jli_strcmp(arg, "-debug") == 0) { addoption("-xdebug", null); } else if (jli_strcmp(arg, "-noclassgc") == 0) { addoption("-xnoclassgc", null); } else if (jli_strcmp(arg, "-xfuture") == 0) { addoption("-xverify:all", null); } else if (jli_strcmp(arg, "-verify") == 0) { addoption("-xverify:all", null); } else if (jli_strcmp(arg, "-verifyremote") == 0) { addoption("-xverify:remote", null); } else if (jli_strcmp(arg, "-noverify") == 0) { addoption("-xverify:none", null); } else if (jli_strccmp(arg, "-prof") == 0) { char *p = arg + 5; char *tmp = jli_memalloc(jli_strlen(arg) + 50); if (*p) { sprintf(tmp, "-xrunhprof:cpu=old,file=%s", p + 1); } else { sprintf(tmp, "-xrunhprof:cpu=old,file=java.prof"); } addoption(tmp, null); } else if (jli_strccmp(arg, "-ss") == 0 || jli_strccmp(arg, "-oss") == 0 || jli_strccmp(arg, "-ms") == 0 || jli_strccmp(arg, "-mx") == 0) { char *tmp = jli_memalloc(jli_strlen(arg) + 6); sprintf(tmp, "-x%s", arg + 1); /* skip '-' */ addoption(tmp, null); } else if (jli_strcmp(arg, "-checksource") == 0 || jli_strcmp(arg, "-cs") == 0 || jli_strcmp(arg, "-noasyncgc") == 0) { /* no longer supported */ jli_reporterrormessage(arg_warn, arg); } else if (jli_strccmp(arg, "-version:") == 0 || jli_strcmp(arg, "-no-jre-restrict-search") == 0 || jli_strcmp(arg, "-jre-restrict-search") == 0 || jli_strccmp(arg, "-splash:") == 0) { ; /* ignore machine independent options already handled */ } else if (processplatformoption(arg)) { ; /* processing of platform dependent options */ } else if (removableoption(arg)) { ; /* do not pass option to vm. */ } else { addoption(arg, null); } } if (--argc >= 0) { *pwhat = *argv++; } if (*pwhat == null) { *pret = 1; } else if (mode == lm_unknown) { /* default to lm_class if -jar and -cp option are * not specified */ mode = lm_class; } if (argc >= 0) { *pargc = argc; *pargv = argv; } *pmode = mode; return jni_true;}/* * inject the -dsun.java.command pseudo property into the args structure * this pseudo property is used in the hotspot vm to expose the * java class name and arguments to the main method to the vm. the * hotspot vm uses this pseudo property to store the java class name * (or jar file name) and the arguments to the class's main method * to the instrumentation memory region. the sun.java.command pseudo * property is not exported by hotspot to the java layer. */voidsetjavacommandlineprop(char *what, int argc, char **argv){ int i = 0; size_t len = 0; char* javacommand = null; char* dashdstr = "-dsun.java.command="; if (what == null) { /* unexpected, one of these should be set. just return without * setting the property */ return; } /* determine the amount of memory to allocate assuming * the individual components will be space separated */ len = jli_strlen(what); for (i = 0; i < argc; i++) { len += jli_strlen(argv[i]) + 1; } /* allocate the memory */ javacommand = (char*) jli_memalloc(len + jli_strlen(dashdstr) + 1); /* build the -d string */ *javacommand = '\0'; jli_strcat(javacommand, dashdstr); jli_strcat(javacommand, what); for (i = 0; i < argc; i++) { /* the components of the string are space separated. in * the case of embedded white space, the relationship of * the white space separated components to their true * positional arguments will be ambiguous. this issue may * be addressed in a future release. */ jli_strcat(javacommand, " "); jli_strcat(javacommand, argv[i]); } addoption(javacommand, null);}// 设置 classpathstatic voidsetclasspath(const char *s){ char *def; const char *orig = s; static const char format[] = "-djava.class.path=%s"; /* * usually we should not get a null pointer, but there are cases where * we might just get one, in which case we simply ignore it, and let the * caller deal with it */ if (s == null) return; s = jli_wildcardexpandclasspath(s); if (sizeof(format) - 2 + jli_strlen(s) < jli_strlen(s)) // s is corrupted after wildcard expansion return; def = jli_memalloc(sizeof(format) - 2 /* strlen("%s") */ + jli_strlen(s)); sprintf(def, format, s); addoption(def, null); if (s != orig) jli_memfree((char *) s);} |
-xxxxx, --xxx格式配置，如 -xms1024g, --noclassgc ... 然后解析出来。最后通过addoption()存储起来。
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。
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// java.cjvminit(invocationfunctions* ifn, jlong threadstacksize, int argc, char **argv, int mode, char *what, int ret){ showsplashscreen(); return continueinnewthread(ifn, threadstacksize, argc, argv, mode, what, ret);}/* * displays the splash screen according to the jar file name * and image file names stored in environment variables */voidshowsplashscreen(){ const char *jar_name = getenv(splash_jar_env_entry); const char *file_name = getenv(splash_file_env_entry); int data_size; void *image_data = null; float scale_factor = 1; char *scaled_splash_name = null; if (file_name == null){ return; } scaled_splash_name = dosplashgetscaledimagename( jar_name, file_name, &scale_factor); if (jar_name) { if (scaled_splash_name) { image_data = jli_jarunpackfile( jar_name, scaled_splash_name, &data_size); } if (!image_data) { scale_factor = 1; image_data = jli_jarunpackfile( jar_name, file_name, &data_size); } if (image_data) { dosplashinit(); dosplashsetscalefactor(scale_factor); dosplashloadmemory(image_data, data_size); jli_memfree(image_data); } } else { dosplashinit(); if (scaled_splash_name) { dosplashsetscalefactor(scale_factor); dosplashloadfile(scaled_splash_name); } else { dosplashloadfile(file_name); } } if (scaled_splash_name) { jli_memfree(scaled_splash_name); } dosplashsetfilejarname(file_name, jar_name); /* * done with all command line processing and potential re-execs so * clean up the environment. */ (void)unsetenv(env_entry); (void)unsetenv(splash_file_env_entry); (void)unsetenv(splash_jar_env_entry); jli_memfree(splash_jar_entry); jli_memfree(splash_file_entry);}intcontinueinnewthread(invocationfunctions* ifn, jlong threadstacksize, int argc, char **argv, int mode, char *what, int ret){ /* * if user doesn't specify stack size, check if vm has a preference. * note that hotspot no longer supports jni_version_1_1 but it will * return its default stack size through the init args structure. */ if (threadstacksize == 0) { struct jdk1_1initargs args1_1; memset((void*)&args1_1, 0, sizeof(args1_1)); args1_1.version = jni_version_1_1; ifn->getdefaultjavavminitargs(&args1_1); /* ignore return value */ if (args1_1.javastacksize > 0) { threadstacksize = args1_1.javastacksize; } } { /* create a new thread to create jvm and invoke main method */ javamainargs args; int rslt; args.argc = argc; args.argv = argv; args.mode = mode; args.what = what; args.ifn = *ifn; rslt = continueinnewthread0(javamain, threadstacksize, (void*)&args); /* if the caller has deemed there is an error we * simply return that, otherwise we return the value of * the callee */ return (ret != 0) ? ret : rslt; }} |
看起来，jvm是通过一个新线程去运行应用系统的。在将执行控制权交由java代码后，它的主要作用，就是不停地接收命令，执行命令。从而变成一个真正的执行机器。
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