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在当今的软件开发中,多线程编程已经成为一种不可或缺的技术。Java作为一种广泛使用的编程语言,提供了丰富的多线程支持。本文将详细介绍Java中实现多线程的几种方法,包括继承Thread
类、实现Runnable
接口、使用ExecutorService
、Callable
和Future
、以及Fork/Join
框架等。我们还将探讨线程同步、线程池、线程间通信等高级主题,帮助读者全面掌握Java多线程编程。
多线程是指在一个程序中同时运行多个线程,每个线程执行不同的任务。多线程可以提高程序的并发性,充分利用多核CPU的计算能力,从而提高程序的执行效率。
在Java中,线程的生命周期包括以下几个状态:
Java中的线程可以设置优先级,优先级范围从1(最低)到10(最高)。默认情况下,线程的优先级为5。优先级高的线程有更大的机会被CPU调度执行。
Thread thread = new Thread();
thread.setPriority(Thread.MAX_PRIORITY); // 设置线程优先级为最高
Thread
类继承Thread
类是实现多线程的最简单方法。通过重写run()
方法,可以定义线程执行的任务。
class MyThread extends Thread {
@Override
public void run() {
System.out.println("Thread is running");
}
}
public class Main {
public static void main(String[] args) {
MyThread thread = new MyThread();
thread.start(); // 启动线程
}
}
Runnable
接口实现Runnable
接口是另一种常见的实现多线程的方法。与继承Thread
类相比,实现Runnable
接口更加灵活,因为Java不支持多继承。
class MyRunnable implements Runnable {
@Override
public void run() {
System.out.println("Thread is running");
}
}
public class Main {
public static void main(String[] args) {
Thread thread = new Thread(new MyRunnable());
thread.start(); // 启动线程
}
}
ExecutorService
ExecutorService
是Java提供的一个高级线程管理工具,可以更方便地管理线程池和任务执行。
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class Main {
public static void main(String[] args) {
ExecutorService executor = Executors.newFixedThreadPool(5); // 创建固定大小的线程池
for (int i = 0; i < 10; i++) {
Runnable task = new MyRunnable();
executor.execute(task); // 提交任务
}
executor.shutdown(); // 关闭线程池
}
}
Callable
和Future
Callable
与Runnable
类似,但它可以返回一个结果,并且可以抛出异常。Future
用于获取Callable
任务的执行结果。
import java.util.concurrent.*;
class MyCallable implements Callable<String> {
@Override
public String call() throws Exception {
return "Task completed";
}
}
public class Main {
public static void main(String[] args) throws ExecutionException, InterruptedException {
ExecutorService executor = Executors.newSingleThreadExecutor();
Future<String> future = executor.submit(new MyCallable());
System.out.println(future.get()); // 获取任务执行结果
executor.shutdown();
}
}
Fork/Join
框架Fork/Join
框架是Java 7引入的一个用于并行计算的框架,特别适合处理可以分解为多个子任务的问题。
import java.util.concurrent.RecursiveTask;
import java.util.concurrent.ForkJoinPool;
class MyTask extends RecursiveTask<Integer> {
private final int threshold = 10;
private int start;
private int end;
public MyTask(int start, int end) {
this.start = start;
this.end = end;
}
@Override
protected Integer compute() {
if (end - start <= threshold) {
int sum = 0;
for (int i = start; i <= end; i++) {
sum += i;
}
return sum;
} else {
int mid = (start + end) / 2;
MyTask leftTask = new MyTask(start, mid);
MyTask rightTask = new MyTask(mid + 1, end);
leftTask.fork();
rightTask.fork();
return leftTask.join() + rightTask.join();
}
}
}
public class Main {
public static void main(String[] args) {
ForkJoinPool pool = new ForkJoinPool();
MyTask task = new MyTask(1, 100);
int result = pool.invoke(task);
System.out.println("Result: " + result);
}
}
在多线程环境中,多个线程可能会同时访问共享资源,导致数据不一致的问题。Java提供了synchronized
关键字来实现线程同步。
class Counter {
private int count = 0;
public synchronized void increment() {
count++;
}
public int getCount() {
return count;
}
}
public class Main {
public static void main(String[] args) throws InterruptedException {
Counter counter = new Counter();
Thread t1 = new Thread(() -> {
for (int i = 0; i < 1000; i++) {
counter.increment();
}
});
Thread t2 = new Thread(() -> {
for (int i = 0; i < 1000; i++) {
counter.increment();
}
});
t1.start();
t2.start();
t1.join();
t2.join();
System.out.println("Count: " + counter.getCount());
}
}
除了同步方法,还可以使用同步代码块来实现更细粒度的同步控制。
class Counter {
private int count = 0;
private final Object lock = new Object();
public void increment() {
synchronized (lock) {
count++;
}
}
public int getCount() {
return count;
}
}
ReentrantLock
ReentrantLock
是Java 5引入的一个可重入锁,提供了比synchronized
更灵活的锁机制。
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
class Counter {
private int count = 0;
private final Lock lock = new ReentrantLock();
public void increment() {
lock.lock();
try {
count++;
} finally {
lock.unlock();
}
}
public int getCount() {
return count;
}
}
wait()
和notify()
wait()
和notify()
是Java中用于线程间通信的基本方法。wait()
使当前线程进入等待状态,notify()
唤醒等待的线程。
class SharedResource {
private boolean flag = false;
public synchronized void produce() {
while (flag) {
try {
wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println("Produced");
flag = true;
notify();
}
public synchronized void consume() {
while (!flag) {
try {
wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println("Consumed");
flag = false;
notify();
}
}
public class Main {
public static void main(String[] args) {
SharedResource resource = new SharedResource();
Thread producer = new Thread(() -> {
for (int i = 0; i < 5; i++) {
resource.produce();
}
});
Thread consumer = new Thread(() -> {
for (int i = 0; i < 5; i++) {
resource.consume();
}
});
producer.start();
consumer.start();
}
}
BlockingQueue
BlockingQueue
是一个线程安全的队列,支持阻塞操作,常用于生产者-消费者模型。
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.LinkedBlockingQueue;
class Producer implements Runnable {
private final BlockingQueue<Integer> queue;
public Producer(BlockingQueue<Integer> queue) {
this.queue = queue;
}
@Override
public void run() {
try {
for (int i = 0; i < 10; i++) {
queue.put(i);
System.out.println("Produced: " + i);
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
class Consumer implements Runnable {
private final BlockingQueue<Integer> queue;
public Consumer(BlockingQueue<Integer> queue) {
this.queue = queue;
}
@Override
public void run() {
try {
while (true) {
int value = queue.take();
System.out.println("Consumed: " + value);
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public class Main {
public static void main(String[] args) {
BlockingQueue<Integer> queue = new LinkedBlockingQueue<>(5);
Thread producer = new Thread(new Producer(queue));
Thread consumer = new Thread(new Consumer(queue));
producer.start();
consumer.start();
}
}
线程池可以有效地管理线程的生命周期,减少线程创建和销毁的开销,提高系统的性能和稳定性。
ThreadPoolExecutor
ThreadPoolExecutor
是Java中实现线程池的核心类,提供了丰富的配置选项。
import java.util.concurrent.*;
public class Main {
public static void main(String[] args) {
ThreadPoolExecutor executor = new ThreadPoolExecutor(
2, // 核心线程数
4, // 最大线程数
60, // 空闲线程存活时间
TimeUnit.SECONDS, // 时间单位
new LinkedBlockingQueue<>(10) // 任务队列
);
for (int i = 0; i < 10; i++) {
Runnable task = new MyRunnable();
executor.execute(task);
}
executor.shutdown();
}
}
ScheduledThreadPoolExecutor
ScheduledThreadPoolExecutor
用于执行定时任务或周期性任务。
import java.util.concurrent.*;
public class Main {
public static void main(String[] args) {
ScheduledThreadPoolExecutor executor = new ScheduledThreadPoolExecutor(2);
Runnable task = new MyRunnable();
executor.scheduleAtFixedRate(task, 0, 1, TimeUnit.SECONDS); // 每隔1秒执行一次任务
}
}
ConcurrentHashMap
ConcurrentHashMap
是线程安全的哈希表,支持高并发的读写操作。
import java.util.concurrent.ConcurrentHashMap;
public class Main {
public static void main(String[] args) {
ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
map.put("key1", 1);
map.put("key2", 2);
System.out.println(map.get("key1"));
}
}
CopyOnWriteArrayList
CopyOnWriteArrayList
是线程安全的列表,适合读多写少的场景。
import java.util.concurrent.CopyOnWriteArrayList;
public class Main {
public static void main(String[] args) {
CopyOnWriteArrayList<String> list = new CopyOnWriteArrayList<>();
list.add("item1");
list.add("item2");
System.out.println(list.get(0));
}
}
Java中的线程中断是一种协作机制,通过调用interrupt()
方法可以请求线程中断。
class MyTask implements Runnable {
@Override
public void run() {
while (!Thread.currentThread().isInterrupted()) {
System.out.println("Running");
}
System.out.println("Thread interrupted");
}
}
public class Main {
public static void main(String[] args) throws InterruptedException {
Thread thread = new Thread(new MyTask());
thread.start();
Thread.sleep(1000);
thread.interrupt(); // 中断线程
}
}
线程可以通过检查中断状态或捕获InterruptedException
来处理中断请求。
class MyTask implements Runnable {
@Override
public void run() {
try {
while (!Thread.currentThread().isInterrupted()) {
System.out.println("Running");
Thread.sleep(100);
}
} catch (InterruptedException e) {
System.out.println("Thread interrupted");
}
}
}
ThreadLocal
ThreadLocal
用于创建线程局部变量,每个线程都有自己独立的变量副本。
class MyTask implements Runnable {
private static ThreadLocal<Integer> threadLocal = new ThreadLocal<>();
@Override
public void run() {
threadLocal.set((int) (Math.random() * 100));
System.out.println("ThreadLocal value: " + threadLocal.get());
}
}
public class Main {
public static void main(String[] args) {
Thread t1 = new Thread(new MyTask());
Thread t2 = new Thread(new MyTask());
t1.start();
t2.start();
}
}
线程组用于管理一组线程,可以对组内的线程进行统一操作。
class MyTask implements Runnable {
@Override
public void run() {
System.out.println("Thread is running");
}
}
public class Main {
public static void main(String[] args) {
ThreadGroup group = new ThreadGroup("MyThreadGroup");
Thread t1 = new Thread(group, new MyTask());
Thread t2 = new Thread(group, new MyTask());
t1.start();
t2.start();
group.interrupt(); // 中断组内所有线程
}
}
可以通过设置UncaughtExceptionHandler
来处理线程中未捕获的异常。
class MyTask implements Runnable {
@Override
public void run() {
throw new RuntimeException("Exception in thread");
}
}
public class Main {
public static void main(String[] args) {
Thread thread = new Thread(new MyTask());
thread.setUncaughtExceptionHandler((t, e) -> {
System.out.println("Exception caught: " + e.getMessage());
});
thread.start();
}
}
Java提供了丰富的多线程编程支持,从基本的Thread
类和Runnable
接口,到高级的ExecutorService
、Callable
和Future
,再到Fork/Join
框架,开发者可以根据需求选择合适的多线程实现方式。同时,Java还提供了线程同步、线程间通信、线程池、线程安全集合等工具,帮助开发者编写高效、稳定的多线程程序。掌握这些技术,将使你在并发编程领域游刃有余。
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