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="brush:php;toolbar:false">
* class PrimeThread extends Thread {
* long minPrime;
* PrimeThread(long minPrime) {
* this.minPrime = minPrime;
* }
*
* public void run() {
* // compute primes larger than minPrime
* . . .
* }
* }
* * The following code would then create a thread and start it running: *
** PrimeThread p = new PrimeThread(143); * p.start(); *登录后复制
* The other way to create a thread is to declare a class that
* implements the Runnable interface. That class then
* implements the run method. An instance of the class can
* then be allocated, passed as an argument when creating
* Thread, and started. The same example in this other
* style looks like the following:
*
* class PrimeRun implements Runnable { * long minPrime; * PrimeRun(long minPrime) { * this.minPrime = minPrime; * } * * public void run() { * // compute primes larger than minPrime * . . . * } * } *登录后复制
* The following code would then create a thread and start it running: *
** PrimeRun p = new PrimeRun(143); * new Thread(p).start(); *登录后复制
* 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. *
* 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 {
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();
}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。
//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");
}
}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。
//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;
}
}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();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();
}
}
}
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();
}
}
}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。更多信息请关注PHP中文网其他相关文章！
