如果你看过ZygoteInit.java的main方法可能会对这个类不陌生，在Android8.1之前，其main方法都是类似以下这样：
以下代码基于Android8.0
public static void main(String argv[]) {
ZygoteServer zygoteServer = new ZygoteServer();
// Mark zygote start. This ensures that thread creation will throw
// an error.
ZygoteHooks.startZygoteNoThreadCreation();
try {
...
// 创建server端的socket，name为"zygote"
zygoteServer.registerServerSocket(socketName);
...
if (startSystemServer) {
// 启动SystemServer进程
startSystemServer(abiList, socketName, zygoteServer);
}
Log.i(TAG, "Accepting command socket connections");
// 等待AMS请求
zygoteServer.runSelectLoop(abiList);
zygoteServer.closeServerSocket();
} catch (Zygote.MethodAndArgsCaller caller) {
// 运行MethodAndArgsCaller的run方法
caller.run();
} catch (Throwable ex) {
Log.e(TAG, "System zygote died with exception", ex);
zygoteServer.closeServerSocket();
throw ex;
}
}
其中比较让人疑惑的地方是caller.run();这句，为何一个Exception需要运行？
我们先看下MethodAndArgsCaller这个类的源码：
/**
* Helper exception class which holds a method and arguments and
* can call them. This is used as part of a trampoline to get rid of
* the initial process setup stack frames.
*/
public static class MethodAndArgsCaller extends Exception
implements Runnable {
/** method to call */
private final Method mMethod;
/** argument array */
private final String[] mArgs;
public MethodAndArgsCaller(Method method, String[] args) {
mMethod = method;
mArgs = args;
}
public void run() {
try {
mMethod.invoke(null, new Object[] { mArgs });
} catch (IllegalAccessException ex) {
throw new RuntimeException(ex);
} catch (InvocationTargetException ex) {
Throwable cause = ex.getCause();
if (cause instanceof RuntimeException) {
throw (RuntimeException) cause;
} else if (cause instanceof Error) {
throw (Error) cause;
}
throw new RuntimeException(ex);
}
}
}
这个类的功能比较单一，可以看出这个类是协助反射调用的，调用了其run方法将通过反射调用传入的方法。
这个类继承了Exception类，我们看抛出这个异常的地方（RuntimeInit类中）：
private static void invokeStaticMain(String className, String[] argv, ClassLoader classLoader)
throws Zygote.MethodAndArgsCaller {
Class<?> cl;
try {
// 根据类名查找类
cl = Class.forName(className, true, classLoader);
} catch (ClassNotFoundException ex) {
throw new RuntimeException(
"Missing class when invoking static main " + className,
ex);
}
Method m;
try {
// 找到该类的main方法
m = cl.getMethod("main", new Class[] { String[].class });
} catch (NoSuchMethodException ex) {
throw new RuntimeException(
"Missing static main on " + className, ex);
} catch (SecurityException ex) {
throw new RuntimeException(
"Problem getting static main on " + className, ex);
}
int modifiers = m.getModifiers();
if (! (Modifier.isStatic(modifiers) && Modifier.isPublic(modifiers))) {
throw new RuntimeException(
"Main method is not public and static on " + className);
}
/*
* This throw gets caught in ZygoteInit.main(), which responds
* by invoking the exception's run() method. This arrangement
* clears up all the stack frames that were required in setting
* up the process.
*/
throw new Zygote.MethodAndArgsCaller(m, argv);
}
到这个方法就可以看出，最终找到某个类的main方法和方法需要的参数，将其传入MethodAndArgsCaller这个Exception中，并在catch了这个Exception的地方调用。
那么为什么要使用这种奇技淫巧调用，而不直接调用某个类呢？
其实这个注释已经解释了：
/*
* This throw gets caught in ZygoteInit.main(), which responds
* by invoking the exception's run() method. This arrangement
* clears up all the stack frames that were required in setting
* up the process.
*/
throw new Zygote.MethodAndArgsCaller(m, argv);
通过抛异常然后调用Exception的run方法的方式，可以清除调用过程的堆栈信息。
解释一下，就是这样做之后，调用的堆栈信息会是类似这样：
...
at com.android.server.SystemServer.main(SystemServer.java:175)
at java.lang.reflect.Method.invoke!(Native method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:738)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:628)
我们看到上面异常信息中只有SystemServer.main、MethodAndArgsCaller.run、ZygoteInit.main，而没有中间的调用过程。这样使得每个被ZygoteInit启动的类看起来都像是直接被启动了，而看不到启动前的设置过程，看起来比较清爽。
额外的收获
我下载的源码是Android9.0，发现MethodAndArgsCaller方法已经不再继承Exception类了，而是仅实现了Runnable接口，同时ZygoteInit类的main方法也不再通过catch Exception的方法运行。
我就很奇怪，难道不再需要清除堆栈信息了吗？
我按照Android9.0的代码实现了一遍上述的调用过程，代码如下：
Main2.java
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
public class Main2 {
public static void main(String[] args) {
new Main2().b().run();
}
private Runnable b(){
return a();
}
private Runnable a() {
return findStaticMain("method_invoke.ClassTwo", new String[]{"111111"}, this.getClass().getClassLoader());
}
/**
* Invokes a static "main(argv[]) method on class "className".
* Converts various failing exceptions into RuntimeExceptions, with
* the assumption that they will then cause the VM instance to exit.
*
* @param className Fully-qualified class name
* @param argv Argument vector for main()
* @param classLoader the classLoader to load {@className} with
*/
protected static Runnable findStaticMain(String className, String[] argv,
ClassLoader classLoader) {
Class<?> cl;
try {
cl = Class.forName(className, true, classLoader);
} catch (ClassNotFoundException ex) {
throw new RuntimeException(
"Missing class when invoking static main " + className,
ex);
}
Method m;
try {
m = cl.getMethod("main", new Class[]{String[].class});
} catch (NoSuchMethodException ex) {
throw new RuntimeException(
"Missing static main on " + className, ex);
} catch (SecurityException ex) {
throw new RuntimeException(
"Problem getting static main on " + className, ex);
}
int modifiers = m.getModifiers();
if (!(Modifier.isStatic(modifiers) && Modifier.isPublic(modifiers))) {
throw new RuntimeException(
"Main method is not public and static on " + className);
}
/*
* This throw gets caught in ZygoteInit.main(), which responds
* by invoking the exception's run() method. This arrangement
* clears up all the stack frames that were required in setting
* up the process.
*/
return new MethodAndArgsCaller(m, argv);
}
/**
* Helper class which holds a method and arguments and can call them. This is used as part of
* a trampoline to get rid of the initial process setup stack frames.
*/
static class MethodAndArgsCaller implements Runnable {
/**
* method to call
*/
private final Method mMethod;
/**
* argument array
*/
private final String[] mArgs;
public MethodAndArgsCaller(Method method, String[] args) {
mMethod = method;
mArgs = args;
}
public void run() {
try {
mMethod.invoke(null, new Object[]{mArgs});
} catch (IllegalAccessException ex) {
throw new RuntimeException(ex);
} catch (InvocationTargetException ex) {
Throwable cause = ex.getCause();
if (cause instanceof RuntimeException) {
throw (RuntimeException) cause;
} else if (cause instanceof Error) {
throw (Error) cause;
}
throw new RuntimeException(ex);
}
}
}
}
ClassTwo.java
public class ClassTwo {
public static void main(String[] args) {
System.out.println(args[0]);
try {
// 制造除0异常
System.out.println(1/0);
} catch (InterruptedException e) {
// 输出堆栈信息
e.printStackTrace();
}
}
}
发现其调用链信息同样是被清除了的：
Exception in thread "main" java.lang.ArithmeticException: / by zero
at method_invoke.ClassTwo.main(ClassTwo.java:9)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at method_invoke.Main2$MethodAndArgsCaller.run(Main2.java:93)
at method_invoke.Main2.main(Main2.java:10)
这没有用什么奇技淫巧，也没有额外的堆栈信息，Android哪个catch Exception的操作在搞什么？
我这时以为是Runnable接口有什么魔力，然后自己写了个接口，让MethodAndArgsCaller继承，结果没有什么两样。
也就是说，将所需要的结果封装成一个对象，最终返回到main方法，main方法中调用就可以了--并不会有中间设置对象的堆栈信息被保留。
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