Thread.getId()

记录该方法是因为我在开发基于 Java Agent 的监控时，根据物理机的线程 id 来匹配 JVM 线程转储中的线程时，发现 JVM 中的线程 id 并不等于物理机上的线程 id，本文简要记录。

首先与 Thread.getId() 相关的源码如下：

/*
 * Thread ID
 */
private long tid;

/**
  * Returns the identifier of this Thread.  The thread ID is a positive
  * <tt>long</tt> number generated when this thread was created.
  * The thread ID is unique and remains unchanged during its lifetime.
  * When a thread is terminated, this thread ID may be reused.
  *
  * @return this thread's ID.
  * @since 1.5
  */
public long getId() {
    return tid;
}    

/* For generating thread ID */
private static long threadSeqNumber;

private static synchronized long nextThreadID() {
    return ++threadSeqNumber;
}

/**
  * Initializes a Thread.
  *
  * @param g the Thread group
  * @param target the object whose run() method gets called
  * @param name the name of the new Thread
  * @param stackSize the desired stack size for the new thread, or
  *        zero to indicate that this parameter is to be ignored.
  * @param acc the AccessControlContext to inherit, or
  *            AccessController.getContext() if null
  * @param inheritThreadLocals if {@code true}, inherit initial values for
  *            inheritable thread-locals from the constructing thread
  */
private void init(ThreadGroup g, Runnable target, String name,
                    long stackSize, AccessControlContext acc,
                    boolean inheritThreadLocals) {
    if (name == null) {
        throw new NullPointerException("name cannot be null");
    }

    this.name = name;

    Thread parent = currentThread();
    SecurityManager security = System.getSecurityManager();
    if (g == null) {
        /* Determine if it's an applet or not */

        /* If there is a security manager, ask the security manager
            what to do. */
        if (security != null) {
            g = security.getThreadGroup();
        }

        /* If the security doesn't have a strong opinion of the matter
            use the parent thread group. */
        if (g == null) {
            g = parent.getThreadGroup();
        }
    }

    /* checkAccess regardless of whether or not threadgroup is
        explicitly passed in. */
    g.checkAccess();

    /*
     * Do we have the required permissions?
     */
    if (security != null) {
        if (isCCLOverridden(getClass())) {
            security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
        }
    }

    g.addUnstarted();

    this.group = g;
    this.daemon = parent.isDaemon();
    this.priority = parent.getPriority();
    if (security == null || isCCLOverridden(parent.getClass()))
        this.contextClassLoader = parent.getContextClassLoader();
    else
        this.contextClassLoader = parent.contextClassLoader;
    this.inheritedAccessControlContext =
            acc != null ? acc : AccessController.getContext();
    this.target = target;
    setPriority(priority);
    if (inheritThreadLocals && parent.inheritableThreadLocals != null)
        this.inheritableThreadLocals =
            ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
    /* Stash the specified stack size in case the VM cares */
    this.stackSize = stackSize;

    /* Set thread ID */
    tid = nextThreadID();
}

根据以上源码，可以看出，Java 内部的线程 id 与底层操作系统的线程 id 无关，Java 中在每次创建线程时使用同步方法 nextThreadID 对静态变量 threadSeqNumber 加一的值作为新生成的线程的 id，对于底层平台的线程 id，我们在线程转储中可以看到，其中 nid=0x7dc8 就是使用十六进制表示的底层平台的线程 id。参见：find out the correspondence between the tid/nid of Java threads as shown from jstack/JMX, on HotSpot/Linux · GitHub。

我们利用以上映射关系，实现了基于 top 命令的高 CPU 使用率 JVM 线程定位。