我正在做一些练习作业,并尝试一些虚拟代码,试图更好地理解线程和锁的概念。以下是一段(有时)陷入死锁的代码。
A.java
public class A {
private B b;
public void setB(B b) {
this.b = b;
}
public synchronized void foo(boolean callBar) {
System.out.println("foo");
if (callBar) {
b.bar(false);
}
}
}
B.java
public class B {
private A a;
public void setA(A a) {
this.a = a;
}
public synchronized void bar(boolean callFoo) {
System.out.println("bar");
if (callFoo) {
a.foo(false);
}
}
}
Demo.java
public class Demo {
public static void main(String[] args) {
A a = new A();
B b = new B();
a.setB(b);
b.setA(a);
new Thread(() -> {
a.foo(true);
}).start();
new Thread(() -> {
b.bar(true);
}).start();
}
}
解决方案:我使用了Lock
而不是synchronized
。
A.java
public class A {
private final ReentrantLock lock = new ReentrantLock();
private B b;
public void setB(B b) {
this.b = b;
}
public ReentrantLock lock() {
return lock;
}
public boolean impendingExecute() {
Boolean thisLock = false;
Boolean otherLock = false;
try {
thisLock = lock.tryLock();
otherLock = b.lock().tryLock();
} finally {
if (!(thisLock && otherLock)) {
if (thisLock) {
lock.unlock();
}
if (otherLock) {
b.lock().unlock();
}
}
}
return thisLock && otherLock;
}
public void foo(boolean callBar) {
System.out.println("foo");
if (callBar && impendingExecute()) {
try {
b.bar(false);
} finally {
lock.unlock();
b.lock().unlock();
}
}
}
}
B.java
public class B {
private final ReentrantLock lock = new ReentrantLock();
private A a;
public void setA(A a) {
this.a = a;
}
public ReentrantLock lock() {
return lock;
}
public boolean impendingExecute() {
Boolean thisLock = false;
Boolean otherLock = false;
try {
thisLock = lock.tryLock();
otherLock = a.lock().tryLock();
} finally {
if (!(thisLock && otherLock)) {
if (thisLock) {
lock.unlock();
}
if (otherLock) {
a.lock().unlock();
}
}
}
return thisLock && otherLock;
}
public void bar(boolean callFoo) {
System.out.println("bar");
if (callFoo && impendingExecute()) {
try {
a.foo(false);
} finally {
lock.unlock();
a.lock().unlock();
}
}
}
}
进行上述更改后,代码不会导致死锁。这是实现此功能的正确方法吗(基本上,我希望审查 impendingExecute()
方法。)?另外,(与评论略有不同)我可以遇到这种情况吗?
注意:我已在代码审查上发布了这个问题,但似乎对虚拟代码的审查是偏离主题的。
最佳答案
您可以使用java.util.concurrent.locks.ReentrantLock
。这种设计允许该方法尝试获取两个类的锁,如果失败则释放锁,并在必要时稍后重试。如果您需要尝试直到成功,那么您需要将其放入循环中并以某种策略终止。
while (true) {
if (this.lock.tryLock()) {
try {
if (ba.lock.tryLock()) {
try {
//some logic
break;
} finally {
ba.lock.unlock();
}
}
} finally {
this.lock.unlock();
}
}
int n = number.nextInt(1000);
int TIME = 1000 + n; // 1 second + random delay to prevent livelock
Thread.sleep(TIME);
}
或者您可以使用此解决方案,确保以相同的顺序获取和释放多个锁:
if (compareTo(ba) < 0) {
former = this;
latter = ba;
} else {
former = ba;
latter = this;
}
synchronized (former) {
synchronized (latter) {
//Some logic
}
}
}
关于java - 使用 ReentrantLock 避免死锁,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/44094584/