运行Java -jar somefile.jar时发生了什么（二）

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（6）Java.c中的LoadMainClass

位置jdk/src/share/bin/java.c

该方法负责载入main函数所在的类。

该方法首先载入sun.launcher.LauncherHelper类，然后调用该类的checkAndLoadMain去找到main函数所在的类，相关代码例如以下。

jclass cls = GetLauncherHelperClass(env);//这个函数载入了sun.launcher.LauncherHelper类
NULL_CHECK0(cls);
if (JLI_IsTraceLauncher()) {
start = CounterGet();
}
NULL_CHECK0(mid = (*env)->GetStaticMethodID(env, cls,//找到方法id
"checkAndLoadMain",
"(ZILjava/lang/String;)Ljava/lang/Class;")); str = NewPlatformString(env, name);
result = (*env)->CallStaticObjectMethod(env, cls, mid, USE_STDERR, mode, str);//调用这个静态方法

（7）LauncherHelper.java中的checkAndLoadMain

位置：jdk/src/share/bin/classes/sun/launcher/LauncherHelper.java

Java代码比較直观。我就直接写凝视了

</pre><p><pre name="code" class="java"> public static Class<?> checkAndLoadMain(boolean printToStderr,
int mode,
String what) {
initOutput(printToStderr);
// mode决定是直接传入的主类还是jar文件。即区分java -jar xxx.jar 还是 java xxx.class
String cn = null;
switch (mode) {
case LM_CLASS:
cn = what;
break;
case LM_JAR:
cn = getMainClassFromJar(what);//这个函数的原理是抽取jar中的mainfest文件得到其主类
break;
default:
// should never happen
throw new InternalError("" + mode + ": Unknown launch mode");
}
cn = cn.replace('/', '.');
Class<?> mainClass = null;
try {
mainClass = scloader.loadClass(cn);//使用scloader载入，怎样载入水比較深。这里就不展开说了。} catch (NoClassDefFoundError | ClassNotFoundException cnfe) {
if (System.getProperty("os.name", "").contains("OS X")
&& Normalizer.isNormalized(cn, Normalizer.Form.NFD)) {
try {
// On Mac OS X since all names with diacretic symbols are given as decomposed it
// is possible that main class name comes incorrectly from the command line
// and we have to re-compose it
mainClass = scloader.loadClass(Normalizer.normalize(cn, Normalizer.Form.NFC));
} catch (NoClassDefFoundError | ClassNotFoundException cnfe1) {
abort(cnfe, "java.launcher.cls.error1", cn);
}
} else {
abort(cnfe, "java.launcher.cls.error1", cn);
}
}
// set to mainClass
appClass = mainClass; /*
* Check if FXHelper can launch it using the FX launcher. In an FX app,
* the main class may or may not have a main method, so do this before
* validating the main class.
*/
if (mainClass.equals(FXHelper.class) ||
FXHelper.doesExtendFXApplication(mainClass)) {
// Will abort() if there are problems with the FX runtime
FXHelper.setFXLaunchParameters(what, mode);
return FXHelper.class;
} validateMainClass(mainClass);//这里对主类有效性即可了验证，代码附在后面。
return mainClass;
}

接下来给出了验证主类是否有效的代码。

static void validateMainClass(Class<?> mainClass) {
Method mainMethod;
try {
mainMethod = mainClass.getMethod("main", String[].class);//看看是否存在例如以下签名的方法 main(String[] argvs)
} catch (NoSuchMethodException nsme) {
// invalid main or not FX application, abort with an error
abort(null, "java.launcher.cls.error4", mainClass.getName(),
FXHelper.JAVAFX_APPLICATION_CLASS_NAME);
return; // Avoid compiler issues
} int mod = mainMethod.getModifiers();//取得限定符
if (!Modifier.isStatic(mod)) {//假设不是static的则报错
abort(null, "java.launcher.cls.error2", "static",
mainMethod.getDeclaringClass().getName());
}
if (mainMethod.getReturnType() != java.lang.Void.TYPE) {
abort(null, "java.launcher.cls.error3",//假设不是void的也报错
mainMethod.getDeclaringClass().getName());
}
}