Java标準庫裡常見的公有API确實是沒有擷取目前程序的ID的方法，有時候挺郁悶的，就是需要自己的PID。
于是有各種workaround，其中有很靠譜的通過JNI調用外部的C/C++擴充，然後調用作業系統提供的相應API去擷取PID；也有些不怎麼靠譜的hack。這裡要介紹的就是後者之一，隻在Sun JDK或相容的JDK上有效的方法。
代碼例子如下：
Java代碼
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: "[email protected]"
try {
return Integer.parseInt(name.substring(0, name.indexOf('@')));
} catch (Exception e) {
return -1;
}
}
}
使用Sun JDK裡RuntimeMXBean.getName()方法的實作，可以很輕松的拿到自身的PID。
運作這個程式可以看到類似以下的輸出：
Command prompt代碼
D:\experiment>java ShowOwnPID
pid: 11704 這個時候再跑一個jps來看看目前都有哪些Java程序的話，可以看到：
Command prompt代碼
D:\experiment>jps
7888 Jps
11704 ShowOwnPID 嗯……這PID沒錯。
這PID是哪兒來的呢？先看RuntimeMXBean執行個體的來源，java.lang.management.ManagementFactory：
Java代碼
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();
}
// ...
}
可以看到它依賴了sun.management.ManagementFactory，于是看看對應的實作：
Java代碼
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();
}
// ...
}
這裡可以發現實作RuntimeMXBean接口的是sun.management.RuntimeImpl。它的實作是：
Java代碼
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()傳回的是VMManagement.getVmId()的傳回值，再跟過去看：
Java代碼
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，這裡可以看到getVmId()傳回過來的字元串确實是"[email protected]"形式的。再追下去，看看native一側是如何擷取PID的話：
j2se/src/share/native/sun/management/VMManagementImpl.c
C代碼
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;
} 于是繼續跟，
hotspot/src/share/vm/services/management.cpp
C++代碼
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 接下來os::current_process_id()的實作就是每個作業系統不同的了。
在Linux上是：
hotspot/src/os/linux/vm/os_linux.cpp
C++代碼
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：
C++代碼
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();
// ...
} =================================================================
好吧其實我是在HotSpot的源碼裡搜pid然後慢慢找出JMM代碼裡有調用過os::current_process_id()，然後才一步步向上找到對應的Java API。剛才問畢玄老大有沒有見過在Java代碼裡擷取PID的辦法，才得知原來以前有人總結過幾種辦法 ，其中第一個就是本文提到的這個。嘛，需求是一直有的，這種功能自然是早該有人搗騰過了。
轉自：http://rednaxelafx.iteye.com/blog/716918
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