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法律研究Java 多執行緒中的死鎖
死鎖描述了一種情況，其中兩個或多個執行緒由於互相等待而永遠被阻塞。死鎖會發生在多個執行緒需要相同的鎖但以不同的順序獲取它們的時候。Java 多執行緒程式可能會出現死鎖情況，因為 synchronized 關鍵字會導致執行執行緒在等待與指定物件關聯的鎖或監視器時被阻塞。下面是一個示例。
示例
public class TestThread {
public static Object Lock1 = new Object();
public static Object Lock2 = new Object();
public static void main(String args[]) {
ThreadDemo1 T1 = new ThreadDemo1();
ThreadDemo2 T2 = new ThreadDemo2();
T1.start();
T2.start();
}
private static class ThreadDemo1 extends Thread {
public void run() {
synchronized (Lock1) {
System.out.println("Thread 1: Holding lock 1...");
try { Thread.sleep(10); }
catch (InterruptedException e) {}
System.out.println("Thread 1: Waiting for lock 2...");
synchronized (Lock2) {
System.out.println("Thread 1: Holding lock 1 & 2...");
}
}
}
}
private static class ThreadDemo2 extends Thread {
public void run() {
synchronized (Lock2) {
System.out.println("Thread 2: Holding lock 2...");
try { Thread.sleep(10); }
catch (InterruptedException e) {}
System.out.println("Thread 2: Waiting for lock 1...");
synchronized (Lock1) {
System.out.println("Thread 2: Holding lock 1 & 2...");
}
}
}
}
}當你編譯並執行上述程式時，會遇到一個死鎖情況，以下是程式產生的輸出 −
輸出
Thread 1: Holding lock 1... Thread 2: Holding lock 2... Thread 1: Waiting for lock 2... Thread 2: Waiting for lock 1...
上述程式將永遠掛起，因為這兩個執行緒都無法繼續進行並彼此等待釋放鎖，因此你可以按 CTRL+C 來退出程式。
死鎖解決方案示例
讓我們更改鎖的順序並執行相同的程式，看看這兩個執行緒是否仍然會相互等待 −
示例
public class TestThread {
public static Object Lock1 = new Object();
public static Object Lock2 = new Object();
public static void main(String args[]) {
ThreadDemo1 T1 = new ThreadDemo1();
ThreadDemo2 T2 = new ThreadDemo2();
T1.start();
T2.start();
}
private static class ThreadDemo1 extends Thread {
public void run() {
synchronized (Lock1) {
System.out.println("Thread 1: Holding lock 1...");
try {
Thread.sleep(10);
} catch (InterruptedException e) {}
System.out.println("Thread 1: Waiting for lock 2...");
synchronized (Lock2) {
System.out.println("Thread 1: Holding lock 1 & 2...");
}
}
}
}
private static class ThreadDemo2 extends Thread {
public void run() {
synchronized (Lock1) {
System.out.println("Thread 2: Holding lock 1...");
try {
Thread.sleep(10);
} catch (InterruptedException e) {}
System.out.println("Thread 2: Waiting for lock 2...");
synchronized (Lock2) {
System.out.println("Thread 2: Holding lock 1 & 2...");
}
}
}
}
}因此僅更改鎖的順序即可防止程式進入死鎖情況，並以以下結果完成 −
輸出
Thread 1: Holding lock 1... Thread 1: Waiting for lock 2... Thread 1: Holding lock 1 & 2... Thread 2: Holding lock 1... Thread 2: Waiting for lock 2... Thread 2: Holding lock 1 & 2...
更新時間：19-6-2020
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