在 LoadTimeAgent 类当中，有一个 premain 方法，我们关注两个问题：
- 第一个问题，
Instrumentation是一个接口，它的具体实现是哪个类？ - 第二个问题，是“谁”调用了
LoadTimeAgent.premain()方法的呢？
public static void premain(String agentArgs, Instrumentation inst)
查看 StackTrace
LoadTimeAgent
package lsieun.agent;
import java.lang.instrument.Instrumentation;
public class LoadTimeAgent {
public static void premain(String agentArgs, Instrumentation inst) {
System.out.println("Premain-Class: " + LoadTimeAgent.class.getName());
System.out.println("agentArgs: " + agentArgs);
System.out.println("Instrumentation Class: " + inst.getClass().getName());
Exception ex = new Exception("Exception from LoadTimeAgent");
ex.printStackTrace(System.out);
}
}
运行
$ java -cp ./target/classes/ -javaagent:./target/TheAgent.jar sample.Program
Premain-Class: lsieun.agent.LoadTimeAgent
agentArgs: null
Instrumentation Class: sun.instrument.InstrumentationImpl
java.lang.Exception: Exception from LoadTimeAgent
at lsieun.agent.LoadTimeAgent.premain(LoadTimeAgent.java:11)
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 sun.instrument.InstrumentationImpl.loadClassAndStartAgent(InstrumentationImpl.java:386)
at sun.instrument.InstrumentationImpl.loadClassAndCallPremain(InstrumentationImpl.java:401)
从上面的输出结果中，可以看到 InstrumentationImpl.loadClassAndCallPremain 方法。
InstrumentationImpl
sun.instrument.InstrumentationImpl 实现了 java.lang.instrument.Instrumentation 接口：
public class InstrumentationImpl implements Instrumentation {
}
loadClassAndCallPremain
在 sun.instrument.InstrumentationImpl 类当中，loadClassAndCallPremain 方法的实现非常简单，它直接调用了 loadClassAndStartAgent 方法：
public class InstrumentationImpl implements Instrumentation {
private void loadClassAndCallPremain(String classname, String optionsString) throws Throwable {
loadClassAndStartAgent(classname, "premain", optionsString);
}
}
loadClassAndCallAgentmain
public class InstrumentationImpl implements Instrumentation {
private void loadClassAndCallAgentmain(String classname, String optionsString) throws Throwable {
loadClassAndStartAgent(classname, "agentmain", optionsString);
}
}
loadClassAndStartAgent
在 sun.instrument.InstrumentationImpl 类当中，loadClassAndStartAgent 方法的作用就是通过 Java 反射的机制来对 premain 或 agentmain 方法进行调用。
在 loadClassAndStartAgent 源码中，我们能够看到更多的细节信息：
- 第一步，从自身的方法定义中，去寻找目标方法：先找带有两个参数的方法；如果没有找到，则找带有一个参数的方法。如果第一步没有找到，则进行第二步。
- 第二步，从父类的方法定义中，去寻找目标方法：先找带有两个参数的方法；如果没有找到，则找带有一个参数的方法。
public class InstrumentationImpl implements Instrumentation {
// Attempt to load and start an agent
private void loadClassAndStartAgent(String classname, String methodname, String optionsString) throws Throwable {
ClassLoader mainAppLoader = ClassLoader.getSystemClassLoader();
Class<?> javaAgentClass = mainAppLoader.loadClass(classname);
Method m = null;
NoSuchMethodException firstExc = null;
boolean twoArgAgent = false;
// The agent class must have a premain or agentmain method that
// has 1 or 2 arguments. We check in the following order:
//
// 1) declared with a signature of (String, Instrumentation)
// 2) declared with a signature of (String)
// 3) inherited with a signature of (String, Instrumentation)
// 4) inherited with a signature of (String)
//
// So the declared version of either 1-arg or 2-arg always takes
// primary precedence over an inherited version. After that, the
// 2-arg version takes precedence over the 1-arg version.
//
// If no method is found then we throw the NoSuchMethodException
// from the first attempt so that the exception text indicates
// the lookup failed for the 2-arg method (same as JDK5.0).
try {
m = javaAgentClass.getDeclaredMethod(methodname,
new Class<?>[]{
String.class,
java.lang.instrument.Instrumentation.class
}
);
twoArgAgent = true;
} catch (NoSuchMethodException x) {
// remember the NoSuchMethodException
firstExc = x;
}
if (m == null) {
// now try the declared 1-arg method
try {
m = javaAgentClass.getDeclaredMethod(methodname, new Class<?>[]{String.class});
} catch (NoSuchMethodException x) {
// ignore this exception because we'll try
// two arg inheritance next
}
}
if (m == null) {
// now try the inherited 2-arg method
try {
m = javaAgentClass.getMethod(methodname,
new Class<?>[]{
String.class,
java.lang.instrument.Instrumentation.class
}
);
twoArgAgent = true;
} catch (NoSuchMethodException x) {
// ignore this exception because we'll try
// one arg inheritance next
}
}
if (m == null) {
// finally try the inherited 1-arg method
try {
m = javaAgentClass.getMethod(methodname, new Class<?>[]{String.class});
} catch (NoSuchMethodException x) {
// none of the methods exists so we throw the
// first NoSuchMethodException as per 5.0
throw firstExc;
}
}
// the premain method should not be required to be public,
// make it accessible so we can call it
// Note: The spec says the following:
// The agent class must implement a public static premain method...
setAccessible(m, true);
// invoke the 1 or 2-arg method
if (twoArgAgent) {
m.invoke(null, new Object[]{optionsString, this});
}
else {
m.invoke(null, new Object[]{optionsString});
}
// don't let others access a non-public premain method
setAccessible(m, false);
}
}
总结
本文内容总结如下：
- 第一点，在
premain方法中，Instrumentation接口的具体实现是sun.instrument.InstrumentationImpl类。 - 第二点，查看 Stack Trace，可以看到
sun.instrument.InstrumentationImpl.loadClassAndCallPremain方法对LoadTimeAgent.premain方法进行了调用。