Java虚拟机提供了一套用于调试（JVMDI）和监视（JVMPI）的接口，Java5之后统一为JVMTI： http://docs.oracle.com/javase/1.5.0/docs/guide/jvmti/ 。
其中JVMDI分为三个部分：JVMDI，JDWP和JDI . http://docs.oracle.com/javase/1.4.2/docs/guide/jpda/architecture.html
这篇就是简单的介绍一下怎么使用JDI去监视程序的运行的。
首先假设有一个简单的程序：
package test;
public class Test {
public static void main(String[] args) {
new Thread() {
@Override
public void run() {
Test test = new Test();
while (true) {
try {
sleep(5000);
} catch (InterruptedException e) {
e.printStackTrace();
}
test.printHello();
}
}
}.start();
}
protected void printHello() {
System.out.println("hello");
}
}
程序中，每隔五秒种，printHello()方法就会执行一次。
如果你希望每次printHell()被执行的时候通知你一下，在不修改代码的情况下，你要怎么办？没办法吧？
看看JDI的定义：
JDI - Java Debug Interface
Defines a high-level Java language interface which tool developers can easily use to write remote debugger applications.
所以首先，我们先以远程调试的方式启动上面的Test类：
java -Xdebug -agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=8800 -cp . test.Test
大致就是以socket传输的方式调试Test类，调试的连接端口为8800，并且连接过程不挂起。
一量启动，就可以看到如下的输出：
Listening for transport dt_socket at address: 8800
hello
hello
hello
这样Server被调试端就准备好了，下面就是写监听端了。这里就要用到jdk中提供的JDI接口了。要使用此接口，我们需要在类路径里包含JDK下的tools.jar等包，可以在<JDK>/lib目录下找着。
一、取得连接器
VirtualMachineManager vmm = Bootstrap.virtualMachineManager();
List<AttachingConnector> connectors = vmm.attachingConnectors();
SocketAttachingConnector sac = null;
for (AttachingConnector ac : connectors) {
if (ac instanceof SocketAttachingConnector) {
sac = (SocketAttachingConnector) ac;
break;
}
}
if (sac == null) {
System.out.println("JDI error");
return;
}
二、连接到远程虚拟器
Map arguments = sac.defaultArguments();
Connector.Argument hostArg = (Connector.Argument) arguments.get(HOST);
Connector.Argument portArg = (Connector.Argument) arguments.get(PORT);
hostArg.setValue("127.0.0.1");
portArg.setValue(String.valueOf(8800));
vm = sac.attach(arguments);
三、取得要关注的类和方法
List<ReferenceType> classesByName = vm.classesByName("test.Test");
if (classesByName == null || classesByName.size() == 0) {
System.out.println("No class found");
return;
}
ReferenceType rt = classesByName.get(0);
List<Method> methodsByName = rt.methodsByName("printHello");
if (methodsByName == null || methodsByName.size() == 0) {
System.out.println("No method found");
return;
}
Method method = methodsByName.get(0);
四、注册监听
vm.setDebugTraceMode(VirtualMachine.TRACE_EVENTS);
vm.resume();
EventRequestManager erm = vm.eventRequestManager();
MethodEntryRequest methodEntryRequest = erm.createMethodEntryRequest();
methodEntryRequest.addClassFilter(rt);
methodEntryRequest.setSuspendPolicy(EventRequest.SUSPEND_NONE);
methodEntryRequest.enable();
BreakpointRequest breakpointRequest = erm
.createBreakpointRequest(method.location());
breakpointRequest.setSuspendPolicy(EventRequest.SUSPEND_EVENT_THREAD);
breakpointRequest.enable();
eventLoop();
这里监听每次方法入口的时候，以及在方法上注册一个断点，发一个通知。
四、eventLoop()的实现
private static void eventLoop() throws Exception {
eventQueue = vm.eventQueue();
while (true) {
if (vmExit == true) {
break;
}
eventSet = eventQueue.remove();
EventIterator eventIterator = eventSet.eventIterator();
while (eventIterator.hasNext()) {
Event event = (Event) eventIterator.next();
execute(event);
}
}
}
private static void execute(Event event) throws Exception {
if (event instanceof VMStartEvent) {
System.out.println("VM started");
eventSet.resume();
} else if (event instanceof BreakpointEvent) {
System.out
.println("Reach Method printHello of test.Test");
eventSet.resume();
} else if (event instanceof MethodEntryEvent) {
MethodEntryEvent mee = (MethodEntryEvent) event;
Method method = mee.method();
System.out.println(method.name() + " was Entered!");
eventSet.resume();
} else if (event instanceof VMDisconnectEvent) {
vmExit = true;
} else {
eventSet.resume();
}
}
最后看输出：
[JDI: EventSet: SUSPEND_EVENT_THREAD]
[JDI: Event: MethodEntryEvent@test.Test:23 in thread Thread-0]
[JDI: Event: BreakpointEvent@test.Test:23 in thread Thread-0]
printHello was Entered!
Reach Method printHello of test.Test
[JDI: EventSet: SUSPEND_EVENT_THREAD]
printHello was Entered!
[JDI: Event: MethodEntryEvent@test.Test:23 in thread Thread-0]
[JDI: Event: BreakpointEvent@test.Test:23 in thread Thread-0]
Reach Method printHello of test.Test
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