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Java中的多线程基本介绍

时间：2021-11-24 17:19:49

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Java中的多线程基本介绍

在 Java 中，多线程是指同时执行两个或多个线程以最大限度地利用 CPU 的过程。 Java 中的线程是一个轻量级进程，只需要较少的资源即可创建和共享进程资源。

多线程和多进程用于 Java 中的多任务处理，但我们更喜欢多线程而不是多进程。这是因为线程使用共享内存区域有助于节省内存，而且线程之间的内容切换比进程快一点。

线程的生命周期

线程在其生命周期中必须经历五种状态。此生命周期由 JVM（Java 虚拟机）控制。这些状态是：

NewRunnableRunningNon-Runnable (Blocked)Terminated

New

在这种状态下，一个新线程开始它的生命周期。这也称为新生线程。如果在调用 start() 方法之前创建 Thread 类的实例，则线程处于新创建状态。

Runnable

一个线程在一个新生成的线程启动后变为可运行的。在这种状态下，线程将执行其任务。

Running

当线程调度程序选择线程时，该线程将处于运行状态。

Non-Runnable (Blocked)

该线程在此状态下仍处于活动状态，但目前还没有资格运行。

Terminated

线程因以下原因而终止：

它的 run() 方法要么正常存在，即线程的代码已经执行了程序。或者由于一些不寻常的错误，如分段错误或未处理的异常。

处于终止状态的线程不会消耗 CPU 的任何周期。

Java线程类

Java Thread 类提供了在线程上创建和执行操作的方法和构造函数。 Java 线程类扩展了 Object 类并实现了 Runnable 接口。

Java Thread方法

这些是 Thread 类中可用的方法：

1. public void start()

它开始执行线程，然后在这个 Thread 对象上调用 run()。

示例：

{public void run() {System.out.println("Thread is running..."); } public static void main(String[] args) {StartExp1 thread1=new StartExp1();thread1.start(); } }

输出

Thread is running…

2. public void run()

该线程用于为线程执行操作。如果线程是使用单独的 Runnable 对象构造的，则实例化 run() 方法。

示例

public class RunExp1 implements Runnable {public void run() {System.out.println("Thread is running..."); } public static void main(String args[]) {RunExp1 r1=new RunExp1(); Thread thread1 =new Thread(r1); thread1.start(); } }

输出

Thread is running…

2. public void sleep()

这会在指定的时间内阻塞当前正在运行的线程。

示例

public class SleepExp1 extends Thread {public void run() {for(int i=1;i<5;i++) {try {Thread.sleep(500); }catch(InterruptedException e){System.out.println(e);} System.out.println(i); } } public static void main(String args[]) {SleepExp1 thread1=new SleepExp1(); SleepExp1 thread2=new SleepExp1(); thread1.start(); thread2.start(); } }

输出

4. public static Thread currentThread()

它返回对当前正在运行的线程的引用。

示例

public class CurrentThreadExp extends Thread {public void run() {System.out.println(Thread.currentThread().getName()); } public static void main(String args[]) {CurrentThreadExp thread1=new CurrentThreadExp(); CurrentThreadExp thread2=new CurrentThreadExp(); thread1.start(); thread2.start(); } }

输出

Thread-0

Thread-1

5. public void join()

它会导致当前线程阻塞，直到第二个线程终止或经过指定的毫秒数。

示例

public class JoinExample1 extends Thread {public void run() {for(int i=1; i<=4; i++) {try {Thread.sleep(500); }catch(Exception e){System.out.println(e);} System.out.println(i); } } public static void main(String args[]) {JoinExample1 thread1 = new JoinExample1(); JoinExample1 thread2 = new JoinExample1(); JoinExample1 thread3 = new JoinExample1(); thread1.start(); try {thread1.join(); }catch(Exception e){System.out.println(e);} thread2.start(); thread3.start(); } }

输出

6. public final int getPriority()

它用于检查线程的优先级。创建线程时，会为其分配一些优先级。此优先级由 JVM 或程序员在创建线程时显式分配。

示例

public class JavaGetPriorityExp extends Thread {public void run() {System.out.println("running thread name is:"+Thread.currentThread().getName()); } public static void main(String args[]) {JavaGetPriorityExp t1 = new JavaGetPriorityExp(); JavaGetPriorityExp t2 = new JavaGetPriorityExp(); System.out.println("t1 thread priority : " + t1.getPriority()); System.out.println("t2 thread priority : " + t2.getPriority()); t1.start(); t2.start(); } }

输出

t1 thread priority : 5

t2 thread priority : 5

running thread name is:Thread-0

running thread name is:Thread-1

7. public final void setPriority()

此方法用于更改线程的优先级。每个线程的优先级由 1 到 10 的整数表示。线程的默认优先级为 5。

示例

public class JavaSetPriorityExp1 extends Thread {public void run() {System.out.println("Priority of thread is: "+Thread.currentThread().getPriority()); } public static void main(String args[]) {JavaSetPriorityExp1 t1=new JavaSetPriorityExp1();t1.setPriority(Thread.MAX_PRIORITY); t1.start(); } }

输出

Priority of thread is: 10

8. public final String getName()

线程类的这个方法用于返回线程的名称。我们不能在我们的程序中覆盖这个方法，因为这个方法是final的。

示例

public class GetNameExample extends Thread {public void run() {System.out.println("Thread is running..."); } public static void main(String args[]) {// creating two threads GetNameExample thread1=new GetNameExample(); GetNameExample thread2=new GetNameExample(); System.out.println("Name of thread1: "+ thread1.getName()); System.out.println("Name of thread2: "+thread2.getName()); thread1.start(); thread2.start(); } }

输出

Name of thread1: Thread-0

Name of thread2: Thread-1

Thread is running…

9. public final void setName()

此方法更改线程的名称。

示例

public class SetNameExample extends Thread {public void run() {System.out.println("running..."); } public static void main(String args[]) {SetNameExample thread1=new SetNameExample(); SetNameExample thread2=new SetNameExample(); thread1.start(); thread2.start(); thread1.setName("Kadamb Sachdeva"); thread2.setName("Great learning"); System.out.println("After changing name of thread1: "+thread1.getName()); System.out.println("After changing name of thread2: "+thread2.getName()); } }

输出

After changing name of thread1: Kadamb Sachdeva

After changing name of thread2: Great Learning

running…

10. public long getId()

它返回线程的标识符。线程 ID 是创建线程时生成的数字。此 ID 在其生命周期内无法更改。但是当线程终止时，ID可以被重用。

示例

public class GetIdExample extends Thread {public void run() {System.out.println("running..."); } public static void main(String args[]) {GetIdExample thread1=new GetIdExample(); System.out.println("Name of thread1: "+thread1.getName()); System.out.println("Id of thread1: "+thread1.getId()); thread1.start(); } }

输出

Name of thread1: Thread-0

Id of thread1: 21

running…

11. public final boolean isAlive()

此方法检查线程是否处于活动状态。如果线程类的 start() 方法已被调用且线程尚未死亡，则线程处于活动状态。

示例

public class JavaIsAliveExp extends Thread {public void run() {try {Thread.sleep(300); System.out.println("is run() method isAlive "+Thread.currentThread().isAlive()); } catch (InterruptedException ie) {} } public static void main(String[] args) {JavaIsAliveExp thread1 = new JavaIsAliveExp(); System.out.println("before starting thread isAlive: "+thread1.isAlive()); thread1.start(); System.out.println("after starting thread isAlive: "+thread1.isAlive()); } }

输出

before starting thread isAlive: false

after starting thread isAlive: true

is run() method isAlive true

12. public static void yield()

该方法暂停当前线程的执行以暂时执行其他线程。

示例

public class JavaYieldExp extends Thread {public void run() {for (int i=0; i<3 ; i++) System.out.println(Thread.currentThread().getName() + " in control"); } public static void main(String[]args) {JavaYieldExp thread1 = new JavaYieldExp(); JavaYieldExp thread2 = new JavaYieldExp(); thread1.start(); thread2.start(); for (int i=0; i<3; i++) {thread1.yield(); System.out.println(Thread.currentThread().getName() + " in control"); } } }

输出

main in control

Thread-0 in control

Thread-1 in control

13. public final void suspend()

该方法用于暂时挂起当前正在运行的线程。使用 resume() 方法，您可以恢复挂起的线程。

示例

public class JavaSuspendExp extends Thread {public void run() {for(int i=1; i<5; i++) {try {sleep(500); System.out.println(Thread.currentThread().getName()); }catch(InterruptedException e){System.out.println(e);} System.out.println(i); } } public static void main(String args[]) {JavaSuspendExp thread1=new JavaSuspendExp (); JavaSuspendExp thread2=new JavaSuspendExp (); JavaSuspendExp thread3=new JavaSuspendExp ();thread1.start(); thread2.start(); thread2.suspend(); thread3.start(); } }

输出

Thread-0

Thread-2

Thread-0

Thread-2

Thread-0

Thread-2

Thread-0

Thread-2

14. public final void resume()

此方法用于恢复挂起的线程。它仅与 suspend() 方法一起使用。

public class JavaResumeExp extends Thread {public void run() {for(int i=1; i<5; i++) {try {sleep(500); System.out.println(Thread.currentThread().getName()); }catch(InterruptedException e){System.out.println(e);} System.out.println(i); } } public static void main(String args[]) {JavaResumeExp thread1=new JavaResumeExp (); JavaResumeExp thread2=new JavaResumeExp (); JavaResumeExp thread3=new JavaResumeExp (); thread1.start(); thread2.start(); thread2.suspend();thread3.start(); thread2.resume();} }

输出

Thread-0

Thread-2

Thread-1

Thread-0

Thread-2

Thread-1

Thread-0

Thread-2

Thread-1

Thread-0

Thread-2

Thread-1

15. public final void stop()

顾名思义，此方法用于停止当前正在运行的线程。请记住，一旦线程执行停止，就无法重新启动。

示例

public class JavaStopExp extends Thread {public void run() {for(int i=1; i<5; i++) {try {sleep(500); System.out.println(Thread.currentThread().getName()); }catch(InterruptedException e){System.out.println(e);} System.out.println(i); } } public static void main(String args[]) {JavaStopExp thread1=new JavaStopExp (); JavaStopExp thread2=new JavaStopExp (); JavaStopExp thread3=new JavaStopExp (); thread1.start(); thread2.start(); thread3.stop(); System.out.println("Thread thread3 is stopped"); } }

16. public void destroy()

此线程方法会破坏线程组及其子组。

示例

public class JavaDestroyExp extends Thread {JavaDestroyExp(String threadname, ThreadGroup tg) {super(tg, threadname); start(); } public void run() {for (int i = 0; i < 2; i++) {try {Thread.sleep(10); } catch (InterruptedException ex) {System.out.println("Exception encounterted");} } System.out.println(Thread.currentThread().getName() + " finished executing"); } public static void main(String arg[]) throws InterruptedException, SecurityException {ThreadGroup g1 = new ThreadGroup("Parent thread"); ThreadGroup g2 = new ThreadGroup(g1, "child thread"); JavaDestroyExp thread1 = new JavaDestroyExp("Thread-1", g1); JavaDestroyExp thread2 = new JavaDestroyExp("Thread-2", g1); thread1.join(); thread2.join(); g2.destroy(); System.out.println(g2.getName() + " destroyed"); g1.destroy(); System.out.println(g1.getName() + " destroyed"); } }

输出

Thread-1 finished executing

Thread-2 finished executing

child thread destroyed

Parent thread destroyed

17. public final boolean isDaemon()

此线程方法将检查线程是否为守护线程。如果是守护线程，则返回true，否则返回false。

对于不了解守护线程的人来说，守护线程是在程序结束时不会阻止 Java 虚拟机 (JVM) 退出但线程仍在运行的线程。

示例

public class JavaIsDaemonExp extends Thread {public void run() {//checking for daemon thread if(Thread.currentThread().isDaemon()) {System.out.println("daemon thread work"); } else {System.out.println("user thread work"); } } public static void main(String[] args) {JavaIsDaemonExp thread1=new JavaIsDaemonExp(); JavaIsDaemonExp thread2=new JavaIsDaemonExp(); JavaIsDaemonExp thread3=new JavaIsDaemonExp(); thread1.setDaemon(true); thread1.start(); thread2.start(); thread3.start(); } }

输出

daemon thread work

user thread work

18. public final void setDaemon(boolean on)

线程的这种方法用于标识或标记线程是守护进程还是用户线程。当所有用户线程都死掉时，JVM 会自动终止该线程。

该线程方法必须在线程开始执行之前运行。

示例

public class JavaSetDaemonExp1 extends Thread {public void run() {if(Thread.currentThread().isDaemon()) {System.out.println("daemon thread work"); } else {System.out.println("user thread work"); } } public static void main(String[] args) {JavaSetDaemonExp1 thread1=new JavaSetDaemonExp1(); JavaSetDaemonExp1 thread2=new JavaSetDaemonExp1(); JavaSetDaemonExp1 thread3=new JavaSetDaemonExp1(); thread1.setDaemon(true); thread1.start(); thread2.setDaemon(true); thread2.start(); thread3.start(); } }

输出

daemon thread work

user thread work

19. public void interrupt()

线程的这种方法用于中断当前正在执行的线程。该方法只能在线程处于睡眠或等待状态时调用。

但是如果线程不处于睡眠或等待状态，那么interrupt()方法不会中断线程，而是将中断标志设置为true。

示例

public class JavaInterruptExp1 extends Thread {public void run() {try {Thread.sleep(1000); System.out.println("javatpoint"); }catch(InterruptedException e){throw new RuntimeException("Thread interrupted..."+e); } } public static void main(String args[]) {JavaInterruptExp1 thread1=new JavaInterruptExp1(); thread1.start(); try {thread1.interrupt(); }catch(Exception e){System.out.println("Exception handled "+e);} } }

输出

Exception in thread “Thread-0” java.lang.RuntimeException: Thread interrupted…java.lang.InterruptedException: sleep interrupted at JavaInterruptExp1.run(JavaInterruptExp1.java:10)

20. public boolean isInterrupted()

该线程方法用于测试线程是否被中断。它将内部标志的值作为真或假返回，即如果线程被中断，它将返回真，否则返回假。

示例

public class JavaIsInterruptedExp extends Thread {public void run() {for(int i=1;i<=3;i++) {System.out.println("doing task....: "+i); } } public static void main(String args[])throws InterruptedException {JavaIsInterruptedExp thread1=new JavaIsInterruptedExp(); JavaIsInterruptedExp thread2=new JavaIsInterruptedExp(); thread1.start(); thread2.start(); System.out.println("is thread interrupted..: "+thread1.isInterrupted()); System.out.println("is thread interrupted..: "+thread2.isInterrupted()); thread1.interrupt(); System.out.println("is thread interrupted..: " +thread1.isInterrupted()); System.out.println("is thread interrupted..: "+thread2.isInterrupted()); } }

输出

is thread interrupted…: false

is thread interrupted…: true

is thread interrupted…: false

doing task….: 1

doing task….: 2

doing task….: 3

doing task….: 1

doing task….: 2

doing task….: 3

21. public static boolean interrupted()

该线程方法用于检查当前线程是否被中断。如果要连续调用此线程方法两次，则第二次调用将返回 false。

如果线程的中断状态为真，那么这个线程方法会将其设置为假。

示例

public class JavaInterruptedExp extends Thread {public void run() {for(int i=1;i<=3;i++) {System.out.println("doing task....: "+i); } } public static void main(String args[])throws InterruptedException {JavaInterruptedExp thread1=new JavaInterruptedExp(); JavaInterruptedExp thread2=new JavaInterruptedExp(); thread1.start(); thread2.start(); System.out.println("is thread thread1 interrupted..:"+thread1.interrupted()); thread1.interrupt(); System.out.println("is thread thread1 interrupted..:"+thread1.interrupted()); System.out.println("is thread thread2 interrupted..:"+thread2.interrupted()); } }

输出

is thread thread1 interrupted…: false

is thread thread2 interrupted…: false

doing task….: 1

doing task….: 2

doing task….: 3

doing task….: 1

doing task….: 2

doing task….: 3

22. public static int activeCount()

线程的这个方法用于返回当前执行线程的线程组中的活动线程数。

此线程方法返回的数字只是一个估计数字，因为此方法遍历内部数据结构时线程数会动态变化。

示例

public class JavaActiveCountExp extends Thread {JavaActiveCountExp(String threadname, ThreadGroup tg) {super(tg, threadname); start(); } public void run() {System.out.println("running thread name is:"+Thread.currentThread().getName()); } public static void main(String arg[]) {ThreadGroup g1 = new ThreadGroup("parent thread group"); JavaActiveCountExp thread1 = new JavaActiveCountExp("Thread-1", g1); JavaActiveCountExp thread2 = new JavaActiveCountExp("Thread-2", g1); System.out.println("number of active thread: "+ g1.activeCount()); } }

输出

number of active thread: 2

running thread name is: Thread-1

running thread name is: Thread-2

23. public final void checkAccess()

该线程方法标识当前线程是否具有修改线程的权限。

示例

public class JavaCheckAccessExp extends Thread{public void run() {System.out.println(Thread.currentThread().getName()+" finished executing"); } public static void main(String arg[]) throws InterruptedException, SecurityException {JavaCheckAccessExp thread1 = new JavaCheckAccessExp(); JavaCheckAccessExp thread2 = new JavaCheckAccessExp(); thread1.start(); thread2.start(); thread1.checkAccess(); System.out.println(t1.getName() + " has access"); thread2.checkAccess(); System.out.println(t2.getName() + " has access"); } }

输出

Thread-0 has access

Thread-1 has access

Thread-0 finished executing

Thread-1 finished executing

24. public static boolean holdsLock(Object obj)

此线程方法检查当前执行的线程是否持有指定对象的监视器锁。如果是这样，那么这个线程方法将返回 true。

示例

public class JavaHoldLockExp implements Runnable {public void run() {System.out.println("Currently executing thread is: " + Thread.currentThread().getName()); System.out.println("Does thread holds lock? " + Thread.holdsLock(this)); synchronized (this) {System.out.println("Does thread holds lock? " + Thread.holdsLock(this)); } } public static void main(String[] args) {JavaHoldLockExp g1 = new JavaHoldLockExp(); Thread thread1 = new Thread(g1); thread1.start(); } }

输出

Currently executing thread is: Thread-0

Does thread holds lock? false

Does thread holds lock? true

除此之外，还有多种线程方法用于不同的任务和目的。这些线程方法如下：

public static void dumpStack()public StackTraceElement[] getStackTrace()public static int enumerate(Thread[] tarray)public Thread.State getState()public final ThreadGroup getThreadGroup()public String toString()public final void notify()public final void notifyAll()public void setContextClassLoader(ClassLoader cl)public ClassLoader getContextClassLoader()public static Thread.UncaughtExceptionHandler getDefaultUncaughtExceptionHandler()public static void setDefaultUncaughtExceptionHandler(Thread.UncaughtExceptionHandler eh)

创建线程

在 Java 中使用多线程时，您可以使用两种方式创建线程：

通过扩展 Thread 类通过实现 Runnable 接口

什么是Thread类

Thread 类提供了在线程上创建和执行操作的方法和构造函数。 Thread 类扩展了 Object 类并实现了 Runnable 接口。

Thread类中使用了各种构造函数，但常用的构造函数有：

Thread()Thread(String name)Thread(Runnable r)Thread(Runnable r,String name)

此外，如前所述，有多种线程方法用于不同的目的和任务。

因此，这些构造函数和方法是由 Thread 类提供的，用于在线程上执行各种操作。

Runnable接口是什么

Runnable 接口被实现，其实例旨在由线程执行。它只有一种方法 run()。

public void run()这用于为线程执行操作。

启动一个线程

在 Java 中进行多线程时，要启动一个新创建的线程，使用 start() 方法。

一个新线程开始（使用新的调用堆栈）。线程从 New 状态移动到 Runnable 状态。当线程有机会执行时，它的目标 run() 方法将运行。

通过扩展 Thread 类的 Java Thread 示例

class Multi extends Thread{public void run(){System.out.println("thread is running..."); } public static void main(String args[]){Multi thread1=new Multi(); thread1.start(); } }

输出

thread is running…

通过实现 Runnable 接口的 Java 线程示例

class Multi3 implements Runnable{public void run(){System.out.println("thread is running..."); } public static void main(String args[]){Multi3 m1=new Multi3(); Thread thread1 =new Thread(m1); thread1.start(); } }

输出

thread is running…

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