Java 线程 4

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