构建线程Thread说明
线程是程序中的执行线程，java虚拟机允许应用程序并发的运行多个线程。
每个线程都有一个优先级，高优先级线程的执行优先于低优先级线程。每个线程都可以或不可以标记为一个守护程序。当某个线程中运行的代码创建一个新 Thread 对象时，该新线程的初始优先级被设定为创建线程的优先级，并且当且仅当创建线程是守护线程时，新线程才是守护程序。
当 Java 虚拟机启动时，通常都会有单个非守护线程（它通常会调用某个指定类的 main 方法）。Java 虚拟机会继续执行线程，直到下列任一情况出现时为止： 1.调用了 Runtime 类的 exit 方法，并且安全管理器允许退出操作发生。 2.非守护线程的所有线程都已停止运行，无论是通过从对 run 方法的调用中返回，还是通过抛出一个传播到 run 方法之外的异常。
实现线程的方式，会在后续的章节中介绍
源码参考如下：
/** * A <i>thread</i> is a thread of execution in a program. The Java * Virtual Machine allows an application to have multiple threads of * execution running concurrently. * <p> * Every thread has a priority. Threads with higher priority are * executed in preference to threads with lower priority. Each thread * may or may not also be marked as a daemon. When code running in * some thread creates a new <code>Thread</code> object, the new * thread has its priority initially set equal to the priority of the * creating thread, and is a daemon thread if and only if the * creating thread is a daemon. * <p> * When a Java Virtual Machine starts up, there is usually a single * non-daemon thread (which typically calls the method named * <code>main</code> of some designated class). The Java Virtual * Machine continues to execute threads until either of the following * occurs: * <ul> * <li>The <code>exit</code> method of class <code>Runtime</code> has been * called and the security manager has permitted the exit operation * to take place. * <li>All threads that are not daemon threads have died, either by * returning from the call to the <code>run</code> method or by * throwing an exception that propagates beyond the <code>run</code> * method. * </ul> * <p> * There are two ways to create a new thread of execution. One is to * declare a class to be a subclass of <code>Thread</code>. This * subclass should override the <code>run</code> method of class * <code>Thread</code>. An instance of the subclass can then be * allocated and started. For example, a thread that computes primes * larger than a stated value could be written as follows: * <hr><blockquote><pre> * class PrimeThread extends Thread { * long minPrime; * PrimeThread(long minPrime) { * this.minPrime = minPrime; * } * * public void run() { * // compute primes larger than minPrime * . . . * } * } * </pre></blockquote><hr> * <p> * The following code would then create a thread and start it running: * <blockquote><pre> * PrimeThread p = new PrimeThread(143); * p.start(); * </pre></blockquote> * <p> * The other way to create a thread is to declare a class that * implements the <code>Runnable</code> interface. That class then * implements the <code>run</code> method. An instance of the class can * then be allocated, passed as an argument when creating * <code>Thread</code>, and started. The same example in this other * style looks like the following: * <hr><blockquote><pre> * class PrimeRun implements Runnable { * long minPrime; * PrimeRun(long minPrime) { * this.minPrime = minPrime; * } * * public void run() { * // compute primes larger than minPrime * . . . * } * } * </pre></blockquote><hr> * <p> * The following code would then create a thread and start it running: * <blockquote><pre> * PrimeRun p = new PrimeRun(143); * new Thread(p).start(); * </pre></blockquote> * <p> * Every thread has a name for identification purposes. More than * one thread may have the same name. If a name is not specified when * a thread is created, a new name is generated for it. * <p> * Unless otherwise noted, passing a {@code null} argument to a constructor * or method in this class will cause a {@link NullPointerException} to be * thrown. * * @author unascribed * @see Runnable * @see Runtime#exit(int) * @see #run() * @see #stop() * @since JDK1.0 */publicclass Thread implements Runnable {
需要的信息
在运行线程之前首先要构造一个线程对象，线程对象在构造的时候需要提供线程所需要的属性，如线程所属的线程组、线程优先级、是否是Daemon线程等信息。在new Thread时会调用以下方法进行实例化Thread对象。 初始化代码如下：
/** * 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 */ private void init(ThreadGroup g, Runnable target, String name, long stackSize, AccessControlContext acc) { if (name == null) { throw new NullPointerException("name cannot be null"); } this.name = name; //当前线程作为该线程的父线程 Thread parent = currentThread(); SecurityManager security = System.getSecurityManager(); //线程组的获取：如果传入的参数为空首先获取系统默认的安全组，如果为空获取父线程的安全组 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; //设置daemon 、priority 属性为父线程对应的值 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); //将父线程的InheritableThreadLocal复制过来 if (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 */ //生成线程id（一个long型的字段threadSeqNumber） tid = nextThreadID(); }
结论
一个新构建的Thread对象（new Thread()）,是由其父线程（当前线程）进行空间分配，而子线程继承了父线程的Daemon、优先级和加载资源的contextClassLoader，以及可继承的ThreadLocal，同时会为子线程分配一个线程id。一个可以运行的线程对象完成初始化工作，并且在堆内存中等待运行。
构建的方式继承Thread代码
//方法1通过继承Thread实现class MyThread extends Thread{ //需要实现的方法，该方法执行具体的业务逻辑 @Override public void run() { System.out.println(Thread.currentThread().getName() +" @@@@ MyThread。run（）我是通过继承Thread实现的多线程"); }}
说明
通过Thread源码发现（Thread implements Runnable）发现thread其实也是一个实现了runnable接口的一个实例，它代表一个线程的实例，并且，启动线程的唯一方法就是通过Thread类的start()实例方法。start()方法是一个native方法，它将启动一个新线程，并执行run()方法。这种方式实现多线程很简单，通过自己的类直接extend Thread，并复写run()方法，就可以启动新线程并执行自己定义的run()方法。
其中run()方法的方法体代表了线程需要完成的任务，称之为线程执行体。当创建此线程类对象时一个新的线程得以创建，并进入到线程新建状态。通过调用线程对象引用的start()方法，使得该线程进入到就绪状态，此时此线程并不一定会马上得以执行，这取决于CPU调度时机。
实现接口Runnable代码
//方法2通过实现runnable接口//实现Runnable接口，并重写该接口的run()方法，该run()方法同样是线程执行体，创建Runnable实现类的实例，//并以此实例作为Thread类的target来创建Thread对象，该Thread对象才是真正的线程对象。class MyRunnable implements Runnable{ @Override public void run() { System.out.println(Thread.currentThread().getName()+ " @@@@ MyRunnable。run（）我是通过实现Runnable接口实现的多线程"); }}
使用Callable、Future实现有返回结果的多线程
使用Callable和Future接口创建线程。具体是创建Callable接口的实现类，并实现clall()方法。并使用FutureTask类来包装Callable实现类的对象，且以此FutureTask对象作为Thread对象的target来创建线程。 可返回值的任务必须实现Callable接口，类似的，无返回值的任务必须Runnable接口。执行Callable任务后，可以获取一个Future的对象，在该对象上调用get就可以获取到Callable任务返回的Object了，再结合线程池接口ExecutorService就可以实现传说中有返回结果的多线程了（关于Executor的使用后续的文章中详细介绍。）。
//方法3通过Executor框架实现class MyCallable implements Callable<Integer>{ //需要实现call方法而不是run方法 @Override public Integer call() throws Exception { return 100; }}
启动线程
通过源码分析得出：
1.对象初始化完成之后，通过执行start方法来执行这个线程，并且java虚拟机会调用该线程的run方法执行线程的业务逻辑；
2.调用start方法之后发现会同时有两个线程在执行：当前线程（parent线程【同步告知java虚拟机，只要线程规划器空闲，应立即启动调用start方法的线程】，从调用返回给start方法）和另一个线程（执行其run方法）。
3.并且多次启动一个线程是非法的。特别是当线程已经结束执行后，不能再重新启动。
start方法源码说明如下：
/** * Causes this thread to begin execution; the Java Virtual Machine * calls the <code>run</code> method of this thread. * <p> * The result is that two threads are running concurrently: the * current thread (which returns from the call to the * <code>start</code> method) and the other thread (which executes its * <code>run</code> method). * <p> * It is never legal to start a thread more than once. * In particular, a thread may not be restarted once it has completed * execution. * * @exception IllegalThreadStateException if the thread was already * started. * @see #run() * @see #stop() */ public synchronized void start() { /** * This method is not invoked for the main method thread or "system" * group threads created/set up by the VM. Any new functionality added * to this method in the future may have to also be added to the VM. * * A zero status value corresponds to state "NEW". */ if (threadStatus != 0) throw new IllegalThreadStateException(); /* Notify the group that this thread is about to be started * so that it can be added to the group's list of threads * and the group's unstarted count can be decremented. */ group.add(this); boolean started = false; try { start0(); started = true; } finally { try { if (!started) { group.threadStartFailed(this); } } catch (Throwable ignore) { /* do nothing. If start0 threw a Throwable then it will be passed up the call stack */ } } } private native void start0();
参考代码
public class TestCreateThread { public static void main(String[] args) { Thread myThread = new MyThread(); myThread.setName("myThread"); myThread.start(); Runnable myRunnable = new MyRunnable(); Thread myRunnableThread = new Thread(myRunnable); myRunnableThread.setName("myRunnableThread"); myRunnableThread.start(); Thread myRunnableThread2 = new MyThread(myRunnable); myRunnableThread2.setName("myRunnableThread2"); myRunnableThread2.start(); //执行结果参考如下： //myThread @@@@ MyThread。run（）我是通过继承Thread实现的多线程 //myRunnableThread2 @@@@ MyThread。run（）我是通过继承Thread实现的多线程 //myRunnableThread @@@@ MyRunnable。run（）我是通过实现Runnable接口实现的多线程 //测试callable方法 // 创建MyCallable对象 Callable<Integer> myCallable = new MyCallable(); //使用FutureTask来包装MyCallable对象 FutureTask<Integer> ft = new FutureTask<Integer>(myCallable); //FutureTask对象作为Thread对象的target创建新的线程 Thread thread = new Thread(ft); thread.start();//启用 //获取信息 try { //取得新创建的新线程中的call()方法返回的结果 //当子线程此方法还未执行完毕，ft.get()方法会一直阻塞， //直到call()方法执行完毕才能取到返回值。 int sum = ft.get(); System.out.println("sum = " + sum); } catch (InterruptedException e) { e.printStackTrace(); } catch (ExecutionException e) { e.printStackTrace(); } //使用ExecutorService处理多线程 ExecutorService pool = Executors.newFixedThreadPool(10); Future<Integer> f = pool.submit(myCallable); // 关闭线程池 pool.shutdown(); try { int sum1 = f.get(); System.out.println("sum1 = " + sum1); } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (ExecutionException e) { e.printStackTrace(); } }}
启动线程的注意事项
无论何种方式，启动一个线程，就要给它一个名字！这对排错诊断系统监控有帮助。否则诊断问题时，无法直观知道某个线程的用途。
Thread与Runnable的关系实现关系
Thread实现接口Runnable，并且实现了run方法，代码参考如下:
//如果该线程是使用独立的 Runnable 运行对象构造的，则调用该 Runnable 对象的 run 方法； //否则，该方法不执行任何操作并返回。 //Thread 的子类应该重写该方法。 /** * If this thread was constructed using a separate * <code>Runnable</code> run object, then that * <code>Runnable</code> object's <code>run</code> method is called; * otherwise, this method does nothing and returns. * <p> * Subclasses of <code>Thread</code> should override this method. * * @see #start() * @see #stop() * @see #Thread(ThreadGroup, Runnable, String) */ @Override public void run() { if (target != null) { target.run(); } }}
区别
当执行到Thread类中的run()方法时，会首先判断target是否存在，存在则执行target中的run()方法，也就是实现了Runnable接口并重写了run()方法的类中的run()方法。当时如果该Runnable的子类是通过一个继承Thread的子类（该且重写了run方法），则真正执行的是Thread子类重写的run方法（由于多态的原因）。
以上就是Java并发之线程的使用以及构建启动线程的详细内容，更多请关注其它相关文章！
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