简单来说，Java Platform Debugger Architecture（JPDA）就是Java提供的一套用于开发Java调试工具的规范，任何的JDK实现都需要实现这个规范。JPDA是一个Architecture，它包括了三个不同层次的规范，如下图，
/ |--------------|
/ | VM |
debuggee - ( |--------------| <------- JVMTI - Java VM Tool Interface
\ | back-end |
\ |--------------|
/ |
comm channel - ( | <--------------- JDWP - Java Debug Wire Protocol
\ |
/ |--------------|
/ | front-end |
debugger - ( |--------------| <------- JDI - Java Debug Interface
\ | UI |
\ |--------------|
具体的就不再赘述，可参考下面的参考资料。
假设我们要开发一个调试工具，那我们只需要使用front-end的JDI的API就可以完成。JDI的API在com.sun.jdi包下，相当于是JDI的接口规范了。除了JDK自带的实现外，我在HotSpot的SA中也发现了一个实现。他俩的实现分别是在com.sun.tools.jdi包下和sun.jvm.hotspot.jdi包下。
我们来稍微看下在代码结构上这两个实现的差异，diff -u，
blues@ubuntu:~/Projects/jdk$ tree src/share/classes/com/sun/tools/jdi/ > ~/jdi_jdk blues@ubuntu:~/Projects/hotspot/agent$ tree src/share/classes/sun/jvm/hotspot/jdi/ > ~/jdi_sa blues@ubuntu:~$ diff -u jdi_jdk jdi_sa --- jdi_jdk 2015-01-31 01:29:49.995433328 +0800 +++ jdi_sa 2015-01-31 01:32:21.231773459 +0800 @@ -1,5 +1,4 @@ -src/share/classes/com/sun/tools/jdi/ -├── AbstractLauncher.java +src/share/classes/sun/jvm/hotspot/jdi ├── ArrayReferenceImpl.java ├── ArrayTypeImpl.java ├── BaseLineInfo.java @@ -12,80 +11,51 @@ ├── ClassLoaderReferenceImpl.java ├── ClassObjectReferenceImpl.java ├── ClassTypeImpl.java -├── CommandSender.java ├── ConcreteMethodImpl.java ├── ConnectorImpl.java ├── DoubleTypeImpl.java ├── DoubleValueImpl.java -├── EventQueueImpl.java -├── EventRequestManagerImpl.java -├── EventSetImpl.java ├── FieldImpl.java ├── FloatTypeImpl.java ├── FloatValueImpl.java -├── GenericAttachingConnector.java -├── GenericListeningConnector.java ├── IntegerTypeImpl.java ├── IntegerValueImpl.java ├── InterfaceTypeImpl.java -├── InternalEventHandler.java -├── JDWPException.java ├── JNITypeParser.java +├── JVMTIThreadState.java ├── LineInfo.java -├── LinkedHashMap.java ├── LocalVariableImpl.java ├── LocationImpl.java -├── LockObject.java ├── LongTypeImpl.java ├── LongValueImpl.java -├── META-INF -│ └── services -│ ├── com.sun.jdi.connect.Connector -│ └── com.sun.jdi.connect.spi.TransportService ├── MethodImpl.java ├── MirrorImpl.java ├── MonitorInfoImpl.java ├── NonConcreteMethodImpl.java ├── ObjectReferenceImpl.java -├── ObsoleteMethodImpl.java -├── Packet.java -├── PacketStream.java ├── PrimitiveTypeImpl.java ├── PrimitiveValueImpl.java -├── ProcessAttachingConnector.java -├── RawCommandLineLauncher.java ├── ReferenceTypeImpl.java -├── resources -│ ├── jdi_ja.properties -│ ├── jdi.properties -│ └── jdi_zh_CN.properties +├── SACoreAttachingConnector.java +├── SADebugServerAttachingConnector.java +├── SADebugServer.java +├── SAJDIClassLoader.java +├── SAPIDAttachingConnector.java ├── SDE.java ├── ShortTypeImpl.java ├── ShortValueImpl.java -├── SocketAttachingConnector.java -├── SocketListeningConnector.java -├── SocketTransportService.java ├── StackFrameImpl.java ├── StratumLineInfo.java ├── StringReferenceImpl.java -├── SunCommandLineLauncher.java -├── TargetVM.java -├── ThreadAction.java ├── ThreadGroupReferenceImpl.java -├── ThreadListener.java ├── ThreadReferenceImpl.java ├── TypeComponentImpl.java ├── TypeImpl.java ├── ValueContainer.java ├── ValueImpl.java ├── VirtualMachineImpl.java -├── VirtualMachineManagerImpl.java -├── VirtualMachineManagerService.java -├── VMAction.java -├── VMListener.java ├── VMModifiers.java -├── VMState.java ├── VoidTypeImpl.java └── VoidValueImpl.java -3 directories, 85 files +0 directories, 58 files
下面直接通过一个例子来对JDI有一个更直观的认识。写一个简单的调试器，对目标Java进程打断点，并且输出一个目标进程的变量（这其实就已经是我们平常调试程序要做的事情了不是吗^_^）。
目标进程（debugee）的代码，我们调试的目标就是输出满足条件的变量i，
package me.kisimple.just4fun;
import java.util.Random;
/**
* Created by blues on 31/01/15.
*/
public class Main {
public static void main(String[] args) throws Exception{
Random random = new Random();
while(true) {
int i = random.nextInt(1000);
if(i % 10 == 0) {
// System.out.println(i);
doNothing();
Thread.sleep(5000);
}
}
}
private static void doNothing() {
}
}
调试器（debuger）的代码，
package me.kisimple.just4fun;
import com.sun.jdi.*;
import com.sun.jdi.connect.Connector;
import com.sun.jdi.event.*;
import com.sun.jdi.request.BreakpointRequest;
import com.sun.jdi.request.EventRequestManager;
import com.sun.tools.jdi.SocketAttachingConnector;
import sun.jvm.hotspot.jdi.SAPIDAttachingConnector;
import java.util.List;
import java.util.Map;
/**
* Created by blues on 31/01/15.
*/
public class SimpleDebugger {
private static final String CLASS_NAME = "me.kisimple.just4fun.Main";
private static final int LINE = 16;
private static final String VAR_NAME = "i";
public static void main(String[] args) {
List<Connector> connectors = Bootstrap.virtualMachineManager().allConnectors();
SocketAttachingConnector sac = null;
SAPIDAttachingConnector sapac = null;
for (Connector connector : connectors) {
if(connector instanceof SocketAttachingConnector) {
sac = (SocketAttachingConnector)connector;
} else if(connector instanceof SAPIDAttachingConnector) {
sapac = (SAPIDAttachingConnector)connector;
}
}
boolean sa = System.getProperty("kis.jdi.sa") != null;
try {
// 1. 使用不同SP提供的connector attach到目标VM上面
VirtualMachine vm = null;
if(sa) {
if(args.length != 1) {
// usage();
return;
}
if(sapac != null) {
Map<String, Connector.Argument> defaultArguments = sapac.defaultArguments();
Connector.Argument pidArg = defaultArguments.get("pid"); // SAPIDAttachingConnector#ARG_PID
pidArg.setValue(args[0]);
vm = sapac.attach(defaultArguments);
System.out.println("using sa...");
System.out.println(vm.allThreads());
}
} else {
if(sac != null) {
Map<String, Connector.Argument> defaultArguments = sac.defaultArguments();
Connector.Argument hostArg = defaultArguments.get("hostname"); // SocketAttachingConnector#ARG_HOST
Connector.Argument portArg = defaultArguments.get("port"); // SocketAttachingConnector#ARG_PORT
hostArg.setValue("localhost");
portArg.setValue("8787");
vm = sac.attach(defaultArguments);
}
}
// process = vm.process();
if(vm == null) {
return;
}
// 2. 发送请求告诉目标VM我们需要关心哪些事件
EventRequestManager requestManager = vm.eventRequestManager();
List<ReferenceType> referenceTypes = vm.classesByName(CLASS_NAME);
List<Location> locations = referenceTypes.get(0).locationsOfLine(LINE);
BreakpointRequest breakpointRequest =
requestManager.createBreakpointRequest(locations.get(0));
breakpointRequest.enable();
// 3. 事件监听以及处理
EventQueue eventQueue = vm.eventQueue();
boolean disconnected = false;
while(true) {
if(disconnected) break;
EventSet eventSet = eventQueue.remove();
EventIterator eventIterator = eventSet.eventIterator();
while(eventIterator.hasNext()) {
Event event = eventIterator.nextEvent();
if(event instanceof BreakpointEvent) {
System.out.println("Reach line " + LINE + " of " + CLASS_NAME);
BreakpointEvent breakpointEvent = (BreakpointEvent) event;
ThreadReference threadReference = breakpointEvent.thread();
StackFrame stackFrame = threadReference.frame(0);
LocalVariable localVariable = stackFrame
.visibleVariableByName(VAR_NAME);
Value value = stackFrame.getValue(localVariable);
int i = ((IntegerValue) value).value();
System.out.println("The local variable " + VAR_NAME + " is " + i);
eventSet.resume();
} else if (event instanceof VMDisconnectEvent) {
System.out.println("VM disconnected.");
disconnected = true;
}
}
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
上面注释已经给出了一般使用JDI API的三个步骤，从中也可以看出其实不同的JDI实现，差异主要在于Connector的实现。Connector是通过ServiceLoader机制来加载的，加载的代码在VirtualMachineManagerImpl中，
protected VirtualMachineManagerImpl() {
/*
* Create a top-level thread group
*/
ThreadGroup top = Thread.currentThread().getThreadGroup();
ThreadGroup parent = null;
while ((parent = top.getParent()) != null) {
top = parent;
}
mainGroupForJDI = new ThreadGroup(top, "JDI main");
/*
* Load the connectors
*/
ServiceLoader<Connector> connectorLoader =
ServiceLoader.load(Connector.class, Connector.class.getClassLoader());
Iterator<Connector> connectors = connectorLoader.iterator();
while (connectors.hasNext()) {
Connector connector;
try {
connector = connectors.next();
} catch (ThreadDeath x) {
throw x;
} catch (Exception x) {
System.err.println(x);
continue;
} catch (Error x) {
System.err.println(x);
continue;
}
addConnector(connector);
}
/*
* Load any transport services and encapsulate them with
* an attaching and listening connector.
*/
ServiceLoader<TransportService> transportLoader =
ServiceLoader.load(TransportService.class,
TransportService.class.getClassLoader());
Iterator<TransportService> transportServices =
transportLoader.iterator();
while (transportServices.hasNext()) {
TransportService transportService;
try {
transportService = transportServices.next();
} catch (ThreadDeath x) {
throw x;
} catch (Exception x) {
System.err.println(x);
continue;
} catch (Error x) {
System.err.println(x);
continue;
}
addConnector(GenericAttachingConnector.create(transportService));
addConnector(GenericListeningConnector.create(transportService));
}
// no connectors found
if (allConnectors().size() == 0) {
throw new Error("no Connectors loaded");
}
// Set the default launcher. In order to be compatible
// 1.2/1.3/1.4 we try to make the default launcher
// "com.sun.jdi.CommandLineLaunch". If this connector
// isn't found then we arbitarly pick the first connector.
//
boolean found = false;
List<LaunchingConnector> launchers = launchingConnectors();
for (LaunchingConnector lc: launchers) {
if (lc.name().equals("com.sun.jdi.CommandLineLaunch")) {
setDefaultConnector(lc);
found = true;
break;
}
}
if (!found && launchers.size() > 0) {
setDefaultConnector(launchers.get(0));
}
}
那么这两个实现又分别提供了什么Connector呢？看下各自的jar包就知道了，如下图，
tools.jar与sa-jdi.jar中的com.sun.jdi.connect.Connector文件的内容分别是，
# List all Sun provided connector providers here. If there # are providers that are only available on a particular OS # then prefix the line with #[OS] and they will automatically # uncommented by the build process - see make/jpda/front/Makefile. # com.sun.tools.jdi.SunCommandLineLauncher com.sun.tools.jdi.RawCommandLineLauncher com.sun.tools.jdi.SocketAttachingConnector com.sun.tools.jdi.SocketListeningConnector com.sun.tools.jdi.SharedMemoryAttachingConnector com.sun.tools.jdi.SharedMemoryListeningConnector com.sun.tools.jdi.ProcessAttachingConnector
# SA JDI Connectors sun.jvm.hotspot.jdi.SACoreAttachingConnector sun.jvm.hotspot.jdi.SADebugServerAttachingConnector sun.jvm.hotspot.jdi.SAPIDAttachingConnector
在我们的SimpleDebugger中分别使用了JDK的SocketAttachingConnector与SA的SAPIDAttachingConnector，看类名很容易就知道，一个是通过socket来连接，一个则是通过pid。接下来就跑一跑程序看下。
作为debugee，运行Main需要加上-agentlib:jdwp=transport=dt_socket,server=y,address=8787这个VM option，这样才能通过socket的方式来连接。
$ java7 -agentlib:jdwp=transport=dt_socket,server=y,address=8787 me.kisimple.just4fun.Main Listening for transport dt_socket at address: 8787
java7是个alias，alias java7=‘/usr/lib/jvm/java-7-openjdk-i386/bin/java‘。jps看了下pid是5156。
$ java7 me.kisimple.just4fun.SimpleDebugger java.lang.IndexOutOfBoundsException: Index: 0, Size: 0 at java.util.ArrayList.rangeCheck(ArrayList.java:635) at java.util.ArrayList.get(ArrayList.java:411) at java.util.Collections$UnmodifiableList.get(Collections.java:1211) at me.kisimple.just4fun.SimpleDebugger.main(SimpleDebugger.java:71) $ java7 me.kisimple.just4fun.SimpleDebugger Reach line 16 of me.kisimple.just4fun.Main The local variable i is 930 Reach line 16 of me.kisimple.just4fun.Main The local variable i is 350 Reach line 16 of me.kisimple.just4fun.Main The local variable i is 70
SimpleDebugger的运行很奇怪，第一次跑总会报错，重跑一下就好了，是个bug？暂时没去管它。
上面的输出是使用了JDK的SocketAttachingConnector的结果。是的我们已经写出了一个很简单的调试器^_^ 下面看下SA的SAPIDAttachingConnector，通过传入了一个system property来选择使用它，当然还要把debugee的pid传过去，
$ java7 -Dkis.jdi.sa=true me.kisimple.just4fun.SimpleDebugger 5156 java.io.IOException at sun.jvm.hotspot.jdi.SAPIDAttachingConnector.attach(SAPIDAttachingConnector.java:126) at me.kisimple.just4fun.SimpleDebugger.main(SimpleDebugger.java:47) Caused by: java.lang.reflect.InvocationTargetException at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:606) at sun.jvm.hotspot.jdi.SAPIDAttachingConnector.createVirtualMachine(SAPIDAttachingConnector.java:87) at sun.jvm.hotspot.jdi.SAPIDAttachingConnector.attach(SAPIDAttachingConnector.java:111) ... 1 more Caused by: sun.jvm.hotspot.debugger.DebuggerException: sun.jvm.hotspot.debugger.DebuggerException: Can't attach to the process at sun.jvm.hotspot.debugger.linux.LinuxDebuggerLocal$LinuxDebuggerLocalWorkerThread.execute(LinuxDebuggerLocal.java:152) at sun.jvm.hotspot.debugger.linux.LinuxDebuggerLocal.attach(LinuxDebuggerLocal.java:268) at sun.jvm.hotspot.HotSpotAgent.attachDebugger(HotSpotAgent.java:602) at sun.jvm.hotspot.HotSpotAgent.setupDebuggerLinux(HotSpotAgent.java:565) at sun.jvm.hotspot.HotSpotAgent.setupDebugger(HotSpotAgent.java:338) at sun.jvm.hotspot.HotSpotAgent.go(HotSpotAgent.java:313) at sun.jvm.hotspot.HotSpotAgent.attach(HotSpotAgent.java:157) at sun.jvm.hotspot.jdi.VirtualMachineImpl.createVirtualMachineForPID(VirtualMachineImpl.java:222) ... 7 more Caused by: sun.jvm.hotspot.debugger.DebuggerException: Can't attach to the process at sun.jvm.hotspot.debugger.linux.LinuxDebuggerLocal.attach0(Native Method) at sun.jvm.hotspot.debugger.linux.LinuxDebuggerLocal.access$100(LinuxDebuggerLocal.java:51) at sun.jvm.hotspot.debugger.linux.LinuxDebuggerLocal$1AttachTask.doit(LinuxDebuggerLocal.java:259) at sun.jvm.hotspot.debugger.linux.LinuxDebuggerLocal$LinuxDebuggerLocalWorkerThread.run(LinuxDebuggerLocal.java:127)
报错了，猜测是权限问题（对比下JDK的JDI实现则没有权限问题），sudo看看，
$ sudo java7 -Dkis.jdi.sa=true me.kisimple.just4fun.SimpleDebugger 5156 [sudo] password for blues: Exception in thread "main" java.lang.NoClassDefFoundError: com/sun/jdi/Bootstrap at me.kisimple.just4fun.SimpleDebugger.main(SimpleDebugger.java:24) Caused by: java.lang.ClassNotFoundException: com.sun.jdi.Bootstrap at java.net.URLClassLoader$1.run(URLClassLoader.java:366) at java.net.URLClassLoader$1.run(URLClassLoader.java:355) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(URLClassLoader.java:354) at java.lang.ClassLoader.loadClass(ClassLoader.java:425) at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308) at java.lang.ClassLoader.loadClass(ClassLoader.java:358) ... 1 more
还是不行，环境变量的问题，我是在.bashrc中export CLASSPATH的，sudo之后，应该是一个新的bash。那再换个笨点的方式执行，
$ sudo su root # export CLASSPATH=.:/usr/lib/jvm/java-7-openjdk-i386/jre/lib/rt.jar:/usr/lib/jvm/java-7-openjdk-i386/lib/tools.jar:/usr/lib/jvm/java-7-openjdk-i386/lib/sa-jdi.jar: # /usr/lib/jvm/java-7-openjdk-i386/bin/java -Dkis.jdi.sa=true me.kisimple.just4fun.SimpleDebugger 5156 using sa... [instance of java.lang.Thread(name='JDWP Transport Listener: dt_socket', id=0), instance of java.lang.Thread(name='JDWP Event Helper Thread', id=1), instance of java.lang.Thread(name='Signal Dispatcher', id=2), instance of java.lang.ref.Finalizer$FinalizerThread(name='Finalizer', id=3), instance of java.lang.ref.Reference$ReferenceHandler(name='Reference Handler', id=4), instance of java.lang.Thread(name='main', id=5)] com.sun.jdi.VMCannotBeModifiedException at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:57) at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) at java.lang.reflect.Constructor.newInstance(Constructor.java:526) at java.lang.Class.newInstance(Class.java:379) at sun.jvm.hotspot.jdi.VirtualMachineImpl.throwNotReadOnlyException(VirtualMachineImpl.java:281) at sun.jvm.hotspot.jdi.VirtualMachineImpl.eventRequestManager(VirtualMachineImpl.java:524) at me.kisimple.just4fun.SimpleDebugger.main(SimpleDebugger.java:67)
前面已经成功输出了debugee当前的所有线程（有点像jstack的功能了）。但是后面报错又是咋回事？看了看官网文档和源码，
The VirtualMachine object returned by this connector‘s attach() method is ‘read-only‘. This means that the method:vm.canBeModified() will return false, and that the JDI client should not call any JDI methods that are defined to throw a VMCannotBeModifiedException in this case.
public EventQueue eventQueue() {
throwNotReadOnlyException("VirtualMachine.eventQueue()");
return null;
}
public EventRequestManager eventRequestManager() {
throwNotReadOnlyException("VirtualMachineImpl.eventRequestManager()");
return null;
}
是的，也就是说SA的JDI实现，对debugee的操作是read only的，没有办法去断点，所以如果要实现debuger还是用JDK的JDI实现吧。
最后，列出上述的两点差异，
原文地址：http://blog.csdn.net/kisimple/article/details/43355209
