Sun的JDK里获取当前进程ID的方法（hack）

Java标准库里常见的公有API确实是没有获取当前进程的ID的方法，有时候挺郁闷的，就是需要自己的PID。
于是有各种workaround，其中有很靠谱的通过JNI调用外部的C/C++扩展，然后调用操作系统提供的相应API去获取PID；也有些不怎么靠谱的hack。这里要介绍的就是后者之一，只在Sun JDK或兼容的JDK上有效的方法。

代码例子如下：

import java.lang.management.ManagementFactory;
import java.lang.management.RuntimeMXBean;

public class ShowOwnPID {
    public static void main(String[] args) throws Exception {
        int pid = getPid();
        System.out.println("pid: " + pid);
        System.in.read(); // block the program so that we can do some probing on it
    }

    private static int getPid() {
        RuntimeMXBean runtime = ManagementFactory.getRuntimeMXBean();
        String name = runtime.getName(); // format: "pid@hostname"
        try {
            return Integer.parseInt(name.substring(0, name.indexOf('@')));
        } catch (Exception e) {
            return -1;
        }
    }
}

使用Sun JDK里RuntimeMXBean.getName()方法的实现，可以很轻松的拿到自身的PID。
运行这个程序可以看到类似以下的输出：

D:\experiment>java ShowOwnPID
pid: 11704

这个时候再跑一个jps来看看当前都有哪些Java进程的话，可以看到：

D:\experiment>jps
7888 Jps
11704 ShowOwnPID

嗯……这PID没错。

这PID是哪儿来的呢？先看RuntimeMXBean实例的来源，java.lang.management.ManagementFactory：

package java.lang.management;

// ...

public class ManagementFactory {
    // ...

    /**
     * Returns the managed bean for the runtime system of 
     * the Java virtual machine.
     *
     * @return a {@link RuntimeMXBean} object for the Java virtual machine.

     */
    public static RuntimeMXBean getRuntimeMXBean() {
        return sun.management.ManagementFactory.getRuntimeMXBean();
    }

    // ...
}

可以看到它依赖了sun.management.ManagementFactory，于是看看对应的实现：

package sun.management;

import java.lang.management.*;
// ...
import static java.lang.management.ManagementFactory.*;

public class ManagementFactory {
    private ManagementFactory() {};

    private static VMManagement jvm;

    private static RuntimeImpl runtimeMBean = null;

    public static synchronized RuntimeMXBean getRuntimeMXBean() {
        if (runtimeMBean == null) {
            runtimeMBean = new RuntimeImpl(jvm);
        }
        return runtimeMBean;
    }

    static {
        AccessController.doPrivileged(new LoadLibraryAction("management"));
        jvm = new VMManagementImpl();
    }

    // ...
}

这里可以发现实现RuntimeMXBean接口的是sun.management.RuntimeImpl。它的实现是：

package sun.management;

import java.lang.management.RuntimeMXBean;
// ...

/**
 * Implementation class for the runtime subsystem.
 * Standard and committed hotspot-specific metrics if any.
 *
 * ManagementFactory.getRuntimeMXBean() returns an instance
 * of this class.
 */
class RuntimeImpl implements RuntimeMXBean {
    private final VMManagement jvm;
    private final long vmStartupTime;

    /**
     * Constructor of RuntimeImpl class.
     */
    RuntimeImpl(VMManagement vm) {
        this.jvm = vm;
        this.vmStartupTime = jvm.getStartupTime();
    }

    public String getName() {
        return jvm.getVmId();
    }

    // ...
}

OK，看到getName()返回的是VMManagement.getVmId()的返回值，再跟过去看：

package sun.management;

import java.net.InetAddress;
import java.net.UnknownHostException;
// ...

class VMManagementImpl implements VMManagement {
    // ...

    public String getVmId() {
        int pid = getProcessId();
        String hostname = "localhost";
        try {
            hostname = InetAddress.getLocalHost().getHostName();
        } catch (UnknownHostException e) {
            // ignore
        }

        return pid + "@" + hostname;
    }
    private native int getProcessId();

    // ...
}

OK，这里可以看到getVmId()返回过来的字符串确实是"pid@hostname"形式的。再追下去，看看native一侧是如何获取PID的话：
j2se/src/share/native/sun/management/VMManagementImpl.c

JNIEXPORT jint JNICALL
Java_sun_management_VMManagementImpl_getProcessId
  (JNIEnv *env, jobject dummy)
{
    jlong pid = jmm_interface->GetLongAttribute(env, NULL,
                                                JMM_OS_PROCESS_ID);
    return (jint) pid;
}

于是继续跟，
hotspot/src/share/vm/services/management.cpp

static jlong get_long_attribute(jmmLongAttribute att) {
  switch (att) {
  // ...
  case JMM_OS_PROCESS_ID:
    return os::current_process_id();

  // ...

  default:
    return -1;
  }
}

JVM_ENTRY(jlong, jmm_GetLongAttribute(JNIEnv *env, jobject obj, jmmLongAttribute att))
  if (obj == NULL) {
    return get_long_attribute(att);
  } else {
    // ...
  }
  return -1;
JVM_END

接下来os::current_process_id()的实现就是每个操作系统不同的了。
在Linux上是：
hotspot/src/os/linux/vm/os_linux.cpp

static pid_t _initial_pid = 0;

int os::current_process_id() {

  // Under the old linux thread library, linux gives each thread
  // its own process id. Because of this each thread will return
  // a different pid if this method were to return the result
  // of getpid(2). Linux provides no api that returns the pid
  // of the launcher thread for the vm. This implementation
  // returns a unique pid, the pid of the launcher thread
  // that starts the vm 'process'.

  // Under the NPTL, getpid() returns the same pid as the
  // launcher thread rather than a unique pid per thread.
  // Use gettid() if you want the old pre NPTL behaviour.

  // if you are looking for the result of a call to getpid() that
  // returns a unique pid for the calling thread, then look at the
  // OSThread::thread_id() method in osThread_linux.hpp file

  return (int)(_initial_pid ? _initial_pid : getpid());
}

// this is called _before_ the most of global arguments have been parsed
void os::init(void) {

  // With LinuxThreads the JavaMain thread pid (primordial thread)
  // is different than the pid of the java launcher thread.
  // So, on Linux, the launcher thread pid is passed to the VM
  // via the sun.java.launcher.pid property.
  // Use this property instead of getpid() if it was correctly passed.
  // See bug 6351349.
  pid_t java_launcher_pid = (pid_t) Arguments::sun_java_launcher_pid();

  _initial_pid = (java_launcher_pid > 0) ? java_launcher_pid : getpid();
  // ...
}

在Windows上是：

static int _initial_pid = 0;

int os::current_process_id()
{
  return (_initial_pid ? _initial_pid : _getpid());
}

// this is called _before_ the global arguments have been parsed
void os::init(void) {
  _initial_pid = _getpid();
  // ...
}

=================================================================

好吧其实我是在HotSpot的源码里搜pid然后慢慢找出JMM代码里有调用过os::current_process_id()，然后才一步步向上找到对应的Java API。刚才问毕玄老大有没有见过在Java代码里获取PID的办法，才得知原来[url=http://blog.igorminar.com/2007/03/how-java-application-can-discover-its.html]以前有人总结过几种办法[/url]，其中第一个就是本文提到的这个。嘛，需求是一直有的，这种功能自然是早该有人捣腾过了。