在當今的軟件開發中，多線程編程已經成為一種不可或缺的技術。Java作為一種廣泛使用的編程語言，提供了豐富的多線程支持。本文將詳細介紹Java中實現多線程的幾種方法，包括繼承Thread類、實現Runnable接口、使用ExecutorService、Callable和Future、以及Fork/Join框架等。我們還將探討線程同步、線程池、線程間通信等高級主題，幫助讀者全面掌握Java多線程編程。
多線程是指在一個程序中同時運行多個線程，每個線程執行不同的任務。多線程可以提高程序的并發性，充分利用多核CPU的計算能力，從而提高程序的執行效率。
在Java中，線程的生命周期包括以下幾個狀態：
Java中的線程可以設置優先級，優先級范圍從1（最低）到10（最高）。默認情況下，線程的優先級為5。優先級高的線程有更大的機會被CPU調度執行。
Thread thread = new Thread();
thread.setPriority(Thread.MAX_PRIORITY); // 設置線程優先級為最高
Thread類繼承Thread類是實現多線程的最簡單方法。通過重寫run()方法，可以定義線程執行的任務。
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(); // 啟動線程
}
}
Runnable接口實現Runnable接口是另一種常見的實現多線程的方法。與繼承Thread類相比，實現Runnable接口更加靈活，因為Java不支持多繼承。
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(); // 啟動線程
}
}
ExecutorServiceExecutorService是Java提供的一個高級線程管理工具，可以更方便地管理線程池和任務執行。
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class Main {
public static void main(String[] args) {
ExecutorService executor = Executors.newFixedThreadPool(5); // 創建固定大小的線程池
for (int i = 0; i < 10; i++) {
Runnable task = new MyRunnable();
executor.execute(task); // 提交任務
}
executor.shutdown(); // 關閉線程池
}
}
Callable和FutureCallable與Runnable類似，但它可以返回一個結果，并且可以拋出異常。Future用于獲取Callable任務的執行結果。
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()); // 獲取任務執行結果
executor.shutdown();
}
}
Fork/Join框架Fork/Join框架是Java 7引入的一個用于并行計算的框架，特別適合處理可以分解為多個子任務的問題。
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);
}
}
在多線程環境中，多個線程可能會同時訪問共享資源，導致數據不一致的問題。Java提供了synchronized關鍵字來實現線程同步。
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，還可以使用同步代碼塊來實現更細粒度的同步控制。
class Counter {
private int count = 0;
private final Object lock = new Object();
public void increment() {
synchronized (lock) {
count++;
}
}
public int getCount() {
return count;
}
}
ReentrantLockReentrantLock是Java 5引入的一個可重入鎖，提供了比synchronized更靈活的鎖機制。
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中用于線程間通信的基本方法。wait()使當前線程進入等待狀態，notify()喚醒等待的線程。
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();
}
}
BlockingQueueBlockingQueue是一個線程安全的隊列，支持阻塞操作，常用于生產者-消費者模型。
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();
}
}
線程池可以有效地管理線程的生命周期，減少線程創建和銷毀的開銷，提高系統的性能和穩定性。
ThreadPoolExecutorThreadPoolExecutor是Java中實現線程池的核心類，提供了豐富的配置選項。
import java.util.concurrent.*;
public class Main {
public static void main(String[] args) {
ThreadPoolExecutor executor = new ThreadPoolExecutor(
2, // 核心線程數
4, // 最大線程數
60, // 空閑線程存活時間
TimeUnit.SECONDS, // 時間單位
new LinkedBlockingQueue<>(10) // 任務隊列
);
for (int i = 0; i < 10; i++) {
Runnable task = new MyRunnable();
executor.execute(task);
}
executor.shutdown();
}
}
ScheduledThreadPoolExecutorScheduledThreadPoolExecutor用于執行定時任務或周期性任務。
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秒執行一次任務
}
}
ConcurrentHashMapConcurrentHashMap是線程安全的哈希表，支持高并發的讀寫操作。
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"));
}
}
CopyOnWriteArrayListCopyOnWriteArrayList是線程安全的列表，適合讀多寫少的場景。
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中的線程中斷是一種協作機制，通過調用interrupt()方法可以請求線程中斷。
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(); // 中斷線程
}
}
線程可以通過檢查中斷狀態或捕獲InterruptedException來處理中斷請求。
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");
}
}
}
ThreadLocalThreadLocal用于創建線程局部變量，每個線程都有自己獨立的變量副本。
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();
}
}
線程組用于管理一組線程，可以對組內的線程進行統一操作。
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(); // 中斷組內所有線程
}
}
可以通過設置UncaughtExceptionHandler來處理線程中未捕獲的異常。
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提供了豐富的多線程編程支持，從基本的Thread類和Runnable接口，到高級的ExecutorService、Callable和Future，再到Fork/Join框架，開發者可以根據需求選擇合適的多線程實現方式。同時，Java還提供了線程同步、線程間通信、線程池、線程安全集合等工具，幫助開發者編寫高效、穩定的多線程程序。掌握這些技術，將使你在并發編程領域游刃有余。
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