Java簡單鎖機制,synchronized死鎖並解決
阿新 • • 發佈:2019-02-16
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();
}
}
/**
*
* 兩個Resource物件 r1, r2
* 兩個執行緒t1, t2
* t1呼叫cooperate(r2)時獲取r1的鎖,如果此時此刻t2正在呼叫cooperate(r1)將獲取r2的鎖
* 這時,t1.cooperate(r2)執行r2.doSome(),將申請獲取r2的鎖,r2鎖被t2獲取,所以等待
* 並且,t2.cooperate(r1)執行r1.doSome(),將申請獲取r1的鎖,r1的鎖被t1獲取,繼續等待,
*
* 結果就是t1 等 t2 的鎖 t2 等 t1 的鎖,互相等待對方的鎖,造成死鎖
*/
public class Demo_deadLock {
public static void main(String[] args) {
Resource r1 = new Resource("1", 1);
Resource r2 = new Resource("2", 2);
// 迴圈次數多一些,為了得到上述情況
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");
}
}
解決方案:
import java.util.concurrent.locks.ReentrantLock;
/**
* Created by butter on 16-11-17.
*/
/**
*
* 每次執行緒嘗試獲取lock,獲取不到,釋放已有lock,解決deadlock
*
*/
class Resource1{
private String name;
private int resource;
private ReentrantLock lock = new ReentrantLock(); //Lock介面
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){
/*嘗試獲取鎖, 獲取不到->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();
}
}