Java Attach API

1. instrucment与Attach API

JDK5中增加了一个包java.lang.instrucment，能够对JVM底层组件进行访问。在JDK 5中，Instrument 要求在运行前利用命令行参数或者系统参数来设置代理类，在实际的运行之中，虚拟机在初始化之时(在绝大多数的 Java 类库被载入之前)，instrumentation的设置已经启动，并在虚拟机中设置了回调函数，检测特定类的加载情况，并完成实际工作
​在Java5中，开发基于Instrucment的应用，需要以下几个步骤

1. 编写premain函数
2. ​jar文件打包
3. ​运行agent

但是在实际的很多的情况下，我们没有办法在虚拟机启动之时就为其设定代理，这样实际上限制了instrument的应用。而Java SE 6的新特性改变了这种情况，通过Java Tool API中的attach方式，我们可以很方便地在运行过程中动态地设置加载代理类，以达到instrumentation的目的
​在JDK6中，针对这点做了改进，开发者可以在main开始执行以后，再开启自己的Instrucment程序
Attach API不是Java的标准API，而是Sun公司提供的一套扩展API，用来向目标JVM"附着"(Attach)代理工具程序的。有了它，开发者可以方便的监控一个JVM，运行一个外加的代理程序，Sun JVM Attach API功能上非常简单，仅提供了如下几个功能

1. 列出当前所有的JVM实例描述
2. Attach到其中一个JVM上，建立通信管道
3. 让目标JVM加载Agent

Relevant Link:

http://iamzhongyong.iteye.com/blog/1843558

 

2. BTrace: VM Attach的两种方式

BTrace的特点之一就是可以动态Attach到一个运行的JVM进程上，然后根据BTrace脚本来对目标JVM进行相应的操作
JVM的 Attach有两种方式

1. 指定javaagent参数
2. 运行时动态attach

0x1: 指定javaagent参数

这种方式的特点就是在目标JVM启动时，就确定好了要加载什么样的代理对象，例如

java -javaagent:xxxx.jar TestMain

TestMain.java

package test;

public class TestMain 
{ 
    public static void main(String[] args) throws InterruptedException
    {
        System.out.println("Hello");
    }

}

TestAgent.java

package test;

import java.lang.instrument.Instrumentation;
import java.io.*;

public class TestMain 
{ 
    public static void agentmain(String args, Instrumentation inst) throws Exception 
    {
        System.out.println("Args:" + args);
    }
    
    public static void premain(String args, Instrumentation inst) throws Exception 
    {
        System.out.println("Pre Args:" + args);
        Class[] classes = inst.getAllLoadedClasses();
        for (Class clazz : classes) 
        {
           System.out.println(clazz.getName());
        }
    } 
}

TestAgent类比较简单，最终它会在目标类的Main方法执行之前，执行premain方法，其主要动作是将以及加载的类打印出来。 我们需要将这个类打包成jar文件，以便在目标JVM启动时候，以参数形式指定给它。打成jar的同时，设定MANIFEST.MF文件的内容。告知目标JVM该如何处理

Agent-Class: TestAgent
Premain-Class: TestAgent
Can-Redine-Classes: true
Can-Retransform-Classes: true

用jar命令将TestAgent打包

1. 编译TestAgent
javac TestAgent.java

2. jar打包
jar cvmf MANIFEST.MF xxx.jar TestAgent.class

启动TestMain，并设置javaagent参数

1. 编译TestMain
javac TestMain.java 

2. 启动TestMain
java -javaagent:xxx.jar TestMain

0x2: 动态Attach，load指定Agent

这种方式与之前指定参数的不同在于，其可以在JVM已经运行的情况下，动态的附着上去，并可以动态加载agent
TestMain.java

public class TestMain 
{
    public static void main(String[] args) throws InterruptedException 
    {  
        while(true)
        {  
            Thread.sleep(10000);  
            new Thread(new WaitThread()).start();  
        }  
    }  
      
   static class WaitThread implements Runnable 
   {  
        @Override  
        public void run() 
        {  
            System.out.println("Hello"); 
        }       
   }  
}

TestAgent.java

import java.lang.instrument.Instrumentation;
import java.io.*;

public class TestAgent
{ 
    public static void agentmain(String args, Instrumentation inst) throws Exception 
    {
        System.out.println("Args:" + args);
    }
    
    public static void premain(String args, Instrumentation inst) throws Exception 
    {
        System.out.println("Pre Args:" + args);
        Class[] classes = inst.getAllLoadedClasses();
        for (Class clazz : classes) 
        {
           System.out.println(clazz.getName());
        }
    } 
}

动态加载agent的情况下，被调用的是agentmain方法, 其会在JVMload的时候，被调用
MANIFEST.MF

Agent-Class: TestAgent
Premain-Class: TestAgent
Can-Redine-Classes: true
Can-Retransform-Classes: true

将类打包为jar包

1. 编译TestAgent
javac TestAgent.java

2. jar打包
jar cvmf MANIFEST.MF xxx.jar TestAgent.class

动态附着到对应的JVM需要使用到JDK的Attach API
Main.java

import com.sun.tools.attach.VirtualMachine;

public class Main 
{  
  public static void main(String[] args) throws Exception
  {  
    VirtualMachine vm = null;  
    String agentjarpath = "C:/Users/zhenghan.zh/Desktop/新建文件夹/xxx.jar"; //agentjar路径  
    vm = VirtualMachine.attach("9730");//目标JVM的进程ID（PID）  
    vm.loadAgent(agentjarpath, "This is Args to the Agent.");  
    vm.detach();  
  }  
}

一旦运行这个Main方法， 其就会动态的附着到我们对应的JVM进程中，并为目标JVM加载我们指定的Agent，以达到我们想做的事情， 比如BTrace就为在附着到目标JVM后，开启一个ServerSocket，以便达到与目标进程通讯的目的

Relevant Link:

http://ivanzhangwb.github.io/btrace-vm-attach-api/ 

 

3. Sun JVM Attach API

Sun JVM Attach API是Sun JVM中的一套非标准的可以连接到JVM上的API，从JDK6开始引入，除了Solaris平台的Sun JVM支持远程的Attach，在其他平台都只允许Attach到本地的JVM上

0x1: 列出当前所有的JVM实例描述

package test;
import java.util.List;

import com.sun.tools.attach.VirtualMachine;
import com.sun.tools.attach.VirtualMachineDescriptor;

public class Test 
{

    public static void main(String[] args) 
    {
        List<VirtualMachineDescriptor> list = VirtualMachine.list();  
        for (VirtualMachineDescriptor vmd : list)  
        {  
            System.out.println("pid:" + vmd.id() + ":" + vmd.displayName());  
        }  
    }

}
//tools.jar needs to be added to the IDE's library path and the program's classpath. The tools.jar file is found in the JDK's lib directory.

0x2: Attach到特定进程的JVM上，并加载Agent

//Attach到JVM上
VirtualMachine virtualmachine = VirtualMachine.attach(pid);  
//加载Agent
String javaHome = virtualmachine.getSystemProperties().getProperty("java.home");  
String agentPath = javaHome + File.separator + "jre" + File.separator + "lib" + File.separator + "management-agent.jar");  
File file = new File(agentPath);  
if(!file.exists())  
{  
     agentPath = javaHome + File.separator + "lib" + File.separator + "management-agent.jar";  
      file = new File(agentPath);  
      if(!file.exists())  
          throw new IOException("Management agent not found");  
      }  
}  
  
agentPath = file.getCanonicalPath();  
try  
{  
     virtualmachine.loadAgent(agentPath, "com.sun.management.jmxremote");  
}  
catch(AgentLoadException e)  
{  
     throw new IOException(e);  
}  
catch(AgentInitializationException agentinitializationexception)  
{  
     throw new IOException(e);  
}  
Properties properties = virtualmachine.getAgentProperties();  
address = (String)properties.get("com.sun.management.jmxremote.localConnectorAddress");  
virtualmachine.detach(); 

0x3: Attach API底层实现(windows)

\openjdk\jdk\src\windows\classes\sun\tools\attach\WindowsAttachProvider.java

public VirtualMachine attachVirtualMachine(String vmid) throws AttachNotSupportedException, IOException
{
    checkAttachPermission();

    // AttachNotSupportedException will be thrown if the target VM can be determined
    // to be not attachable.
    testAttachable(vmid);

    return new WindowsVirtualMachine(this, vmid);
}

\openjdk\jdk\src\windows\classes\sun\tools\attach\WindowsVirtualMachine.java

WindowsVirtualMachine(AttachProvider provider, String id) throws AttachNotSupportedException, IOException
{
    //继承HotSpotVirtualMachine
    super(provider, id);

    int pid;
    try 
    {
        pid = Integer.parseInt(id);
    } 
    catch (NumberFormatException x) 
    {
        throw new AttachNotSupportedException("Invalid process identifier");
    }
    //先连接上目标JVM
    hProcess = openProcess(pid);

    // The target VM might be a pre-6.0 VM so we enqueue a "null" command
    // which minimally tests that the enqueue function exists in the target
    // VM.
    try 
    {
        enqueue(hProcess, stub, null, null);
    } 
    catch (IOException x) 
    {
        throw new AttachNotSupportedException(x.getMessage());
    }
}

WindowsVirtualMachine继承HotSpotVirtualMachine，先看看HotSpotVirtualMachine的loadAgent方法
\openjdk\jdk\src\share\classes\sun\tools\attach\HotSpotVirtualMachine.java

/*
* Load JPLIS agent which will load the agent JAR file and invoke
* the agentmain method.
*/
public void loadAgent(String agent, String options) throws AgentLoadException, AgentInitializationException, IOException
{
    String args = agent;
    if (options != null) 
    {
        args = args + "=" + options;
    }
    try 
    {
        loadAgentLibrary("instrument", args);
    } 
    catch (AgentLoadException x) 
    {
        throw new InternalError("instrument library is missing in target VM");
    } 
    catch (AgentInitializationException x) 
    {
        /*
         * Translate interesting errors into the right exception and
         * message (FIXME: create a better interface to the instrument
         * implementation so this isn't necessary)
         */
        int rc = x.returnValue();
        switch (rc) 
        {
        case JNI_ENOMEM:
            throw new AgentLoadException("Insuffient memory");
        case ATTACH_ERROR_BADJAR:
            throw new AgentLoadException("Agent JAR not found or no Agent-Class attribute");
        case ATTACH_ERROR_NOTONCP:
            throw new AgentLoadException("Unable to add JAR file to system class path");
        case ATTACH_ERROR_STARTFAIL:
            throw new AgentInitializationException("Agent JAR loaded but agent failed to initialize");
        default :
            throw new AgentLoadException("Failed to load agent - unknown reason: " + rc);
        }
    }
}

loadAgentLibrary("instrument", args);

/*
* Load agent library
* If isAbsolute is true then the agent library is the absolute path
* to the library and thus will not be expanded in the target VM.
* if isAbsolute is false then the agent library is just a library
* name and it will be expended in the target VM.
*/
private void loadAgentLibrary(String agentLibrary, boolean isAbsolute, String options) throws AgentLoadException, AgentInitializationException, IOException
{
    InputStream in = execute("load",
                 agentLibrary,
                 isAbsolute ? "true" : "false",
                 options);
    try 
    {
        int result = readInt(in);
        if (result != 0) 
        {
        throw new AgentInitializationException("Agent_OnAttach failed", result);
        }
    } 
    finally 
    {
        in.close();

    }
}

可以看到，Java在Attach到目标进行后，调用execute让目标进行加载Agent类，我们继续分析execute的实现方式，可以看到，JVM进程间通信是JVM Attach API的核心，JVM自身就预留了执行来自Attach进程的指令接口
\openjdk\jdk\src\windows\classes\sun\tools\attach\WindowsVirtualMachine.java

InputStream execute(String cmd, Object ... args) throws AgentLoadException, IOException
{
    assert args.length <= 3;        // includes null

    // create a pipe using a random name
    int r = (new Random()).nextInt();
    String pipename = "\\\\.\\pipe\\javatool" + r;
    long hPipe = createPipe(pipename);

    // check if we are detached - in theory it's possible that detach is invoked
    // after this check but before we enqueue the command.
    if (hProcess == -1) 
    {
        closePipe(hPipe);
        throw new IOException("Detached from target VM");
    }

    try 
    {
        // enqueue the command to the process
        enqueue(hProcess, stub, cmd, pipename, args);

        // wait for command to complete - process will connect with the
        // completion status
        connectPipe(hPipe);

        // create an input stream for the pipe
        PipedInputStream is = new PipedInputStream(hPipe);

        // read completion status
        int status = readInt(is);
        if (status != 0) 
        {
        // special case the load command so that the right exception is thrown
        if (cmd.equals("load")) 
        {
            throw new AgentLoadException("Failed to load agent library");
        } 
        else 
        {
            throw new IOException("Command failed in target VM");
        }
        }

        // return the input stream
        return is;

    } 
    catch (IOException ioe) 
    {
        closePipe(hPipe);
        throw ioe;
    }
}

JVM的execute方法中调用了大量native方法，并且从代码中可以看出，JVM Attach的进程间通信使用了管道进行通信

Relevant Link:

http://ayufox.iteye.com/blog/655761 
http://docs.oracle.com/javase/7/docs/jdk/api/attach/spec/com/sun/tools/attach/VirtualMachine.html
http://docs.oracle.com/javase/7/docs/jdk/api/attach/spec/index.html