Java 线程 4 - 线程的属性

参考：

Java 线程 0 - 前言

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下面学习 Java 线程属性

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主要内容：

1. Java 属性浅析
2. 线程 id 和线程名
3. 线程优先级
4. 守护线程

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Java 属性浅析

Java 属性包括线程 id，线程名，线程优先级，是否是守护线程等

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线程 id 和线程名

系统赋予每个线程一个单独的 id，所以通过 id 比较可以判断两个线程是否相同

/**
 * 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;
}

而对于线程名而言，不同的线程可以拥有相同的线程名，设置线程名有两种方式：

1. 在创建新线程的时候赋值
2. 使用函数 setName 赋值

setName 实现方法如下：

/**
 * Changes the name of this thread to be equal to the argument
 * <code>name</code>.
 * <p>
 * First the <code>checkAccess</code> method of this thread is called
 * with no arguments. This may result in throwing a
 * <code>SecurityException</code>.
 *
 * @param      name   the new name for this thread.
 * @exception  SecurityException  if the current thread cannot modify this
 *               thread.
 * @see        #getName
 * @see        #checkAccess()
 */
public final synchronized void setName(String name) {
    checkAccess();
    if (name == null) {
        throw new NullPointerException("name cannot be null");
    }

    this.name = name;
    if (threadStatus != 0) {
        setNativeName(name);
    }
}

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线程优先级

Thread 类定义了 3 个优先级常量：

/**
 * The minimum priority that a thread can have.
 */
public final static int MIN_PRIORITY = 1;

/**
* The default priority that is assigned to a thread.
*/
public final static int NORM_PRIORITY = 5;

/**
 * The maximum priority that a thread can have.
 */
public final static int MAX_PRIORITY = 10;

默认线程优先级常量为 NROM_PRIORITY，新建的线程优先级和当前线程优先级相同

使用方法 setPriority 设置优先级，使用方法 getPriority 获取优先级：

/**
 * Changes the priority of this thread.
 * <p>
 * First the <code>checkAccess</code> method of this thread is called
 * with no arguments. This may result in throwing a
 * <code>SecurityException</code>.
 * <p>
 * Otherwise, the priority of this thread is set to the smaller of
 * the specified <code>newPriority</code> and the maximum permitted
 * priority of the thread's thread group.
 *
 * @param newPriority priority to set this thread to
 * @exception  IllegalArgumentException  If the priority is not in the
 *               range <code>MIN_PRIORITY</code> to
 *               <code>MAX_PRIORITY</code>.
 * @exception  SecurityException  if the current thread cannot modify
 *               this thread.
 * @see        #getPriority
 * @see        #checkAccess()
 * @see        #getThreadGroup()
 * @see        #MAX_PRIORITY
 * @see        #MIN_PRIORITY
 * @see        ThreadGroup#getMaxPriority()
 */
public final void setPriority(int newPriority) {
    ThreadGroup g;
    checkAccess();
    if (newPriority > MAX_PRIORITY || newPriority < MIN_PRIORITY) {
        throw new IllegalArgumentException();
    }
    if((g = getThreadGroup()) != null) {
        if (newPriority > g.getMaxPriority()) {
            newPriority = g.getMaxPriority();
        }
        setPriority0(priority = newPriority);
    }
}

/**
 * Returns this thread's priority.
 *
 * @return  this thread's priority.
 * @see     #setPriority
 */
public final int getPriority() {
    return priority;
}

Note 1：默认优先级范围在 [1-10]，但是不同操作系统会有差别，推荐使用优先级常量进行设置

Note 2：当有多个线程就绪时，优先级高的线程先获得处理器资源

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守护线程

守护线程又称为后台线程，精灵线程，比如，Java 的垃圾回收线程是守护线程，main 线程不是守护线程。其特征是前台线程全部死亡，守护线程自动死亡

使用方法 isDaemon 判断是否是守护线程；使用方法 setDaemon 进行设置：

/**
 * Marks this thread as either a {@linkplain #isDaemon daemon} thread
 * or a user thread. The Java Virtual Machine exits when the only
 * threads running are all daemon threads.
 *
 * <p> This method must be invoked before the thread is started.
 *
 * @param  on
 *         if {@code true}, marks this thread as a daemon thread
 *
 * @throws  IllegalThreadStateException
 *          if this thread is {@linkplain #isAlive alive}
 *
 * @throws  SecurityException
 *          if {@link #checkAccess} determines that the current
 *          thread cannot modify this thread
 */
public final void setDaemon(boolean on) {
    checkAccess();
    if (isAlive()) {
        throw new IllegalThreadStateException();
    }
    daemon = on;
}

/**
 * Tests if this thread is a daemon thread.
 *
 * @return  <code>true</code> if this thread is a daemon thread;
 *          <code>false</code> otherwise.
 * @see     #setDaemon(boolean)
 */
public final boolean isDaemon() {
    return daemon;
}

Note 1：方法 setDaemon 调用该方法必须在线程就绪之前，否则会抛出IllegalThreadStateException 异常

Note 2：前台线程创建的默认是前台线程，后台线程创建的默认是后台线程