下面例子有一定概率deadLock
import com.sun.org.apache.regexp.internal.RE;
/**
* Created by butter on 16/11/15.
*/
class Resource{
private String name;
private int resource;
public Resource(String name, int resource) {
this.name = name;
this.resource = resource;
}
synchronized void doSome(){
System.out.println("doing some thing");
resource++;
}
synchronized void cooperate(Resource resource){
System.out.println("cooperating");
resource.doSome();
}
}
/**
*
* 两个Resource对象 r1, r2
* 两个线程t1, t2
* t1调用cooperate(r2)时获取r1的锁，如果此时此刻t2正在调用cooperate(r1)将获取r2的锁
* 这时，t1.cooperate(r2)执行r2.doSome(),将申请获取r2的锁，r2锁被t2获取，所以等待
* 并且，t2.cooperate(r1)执行r1.doSome(),将申请获取r1的锁，r1的锁被t1获取，继续等待，
*
* 结果就是t1 等 t2 的锁 t2 等 t1 的锁，互相等待对方的锁，造成死锁
*/
public class Demo_deadLock {
public static void main(String[] args) {
Resource r1 = new Resource("1", 1);
Resource r2 = new Resource("2", 2);
// 循环次数多一些，为了得到上述情况
Thread thread1 = new Thread(()->{
for (int i = 0; i < 100; i++) {
r1.doSome();
r1.cooperate(r2);
}
});
System.out.println("t1 over");
Thread thread2 = new Thread(()->{
for (int i = 0; i < 100; i++) {
r2.doSome();
r2.cooperate(r1);
}
});
thread1.start();
thread2.start();
System.out.println("t2 over");
}
}
解决方案:
import java.util.concurrent.locks.ReentrantLock;
/**
* Created by butter on 16-11-17.
*/
/**
*
* 每次线程尝试获取lock,获取不到,释放已有lock,解决deadlock
*
*/
class Resource1{
private String name;
private int resource;
private ReentrantLock lock = new ReentrantLock(); //Lock接口
public Resource1(String name, int resource) {
this.name = name;
this.resource = resource;
}
void doSome(){
System.out.println(Thread.currentThread().getName() + " doing some thing: " + name+ " " + resource);
resource++;
}
void cooperate(Resource1 resource){
while(true) {
try {
if (lockMeAnd(resource)) {
System.out.println(Thread.currentThread().getName() + " r1 & r2 has been Locked");
System.out.println(Thread.currentThread().getName() + " cooperating");
resource.doSome();
break;
} else {
System.out.println( Thread.currentThread().getName() +" can't lock r1&r2");
}
}finally {
unLockMeAnd(resource);
System.out.println(Thread.currentThread().getName() + " r1 & r2 has been unLocked");
}
}
}
private boolean lockMeAnd(Resource1 res){
/*尝试获取锁, 获取不到->不阻塞*/
return this.lock.tryLock() && res.lock.tryLock();
}
private void unLockMeAnd(Resource1 res){
if(this.lock.isHeldByCurrentThread()){
this.lock.unlock();
}
if(res.lock.isHeldByCurrentThread()){
res.lock.unlock();
}
}
}
public class Demo_noDeadLock {
public static void main(String[] args) {
Resource1 r1 = new Resource1("a", 1);
Resource1 r2 = new Resource1("b", 2);
Thread t1 = new Thread(()->{
for (int i = 0; i < 10; i++) {
r1.cooperate(r2);
}
});
Thread t2 = new Thread(()->{
for (int i = 0; i < 10; i++) {
r2.cooperate(r1);
}
});
t1.start();
t2.start();
}
}