Java中常見死鎖與活鎖的實例詳解
本文介紹了Java中常見死鎖與活鎖的實例詳解,分享給大家,具體如下:
- 順序死鎖:過度加鎖,導致由于執(zhí)行順序的原因,互相持有對方正在等待的鎖
- 資源死鎖:多個線程在相同的資源上發(fā)生等待
由于調(diào)用順序而產(chǎn)生的死鎖
public class Test {
Object leftLock = new Object();
Object rightLock = new Object();
public static void main(String[] args) {
final Test test = new Test();
Thread a = new Thread(new Runnable() {
@Override public void run() {
int i=0;
while (i<10)
{
test.leftRight();
i++;
}
}
},"aThread");
Thread b = new Thread(new Runnable() {
@Override public void run() {
int i=0;
while (i<10)
{
test.rightleft();
i++;
}
}
},"bThread");
a.start();
b.start();
}
public void leftRight(){
synchronized (leftLock){
System.out.println(Thread.currentThread().getName()+":leftRight:get left");
synchronized (rightLock){
System.out.println(Thread.currentThread().getName()+":leftRight:get right");
}
}
}
public void rightleft(){
synchronized (rightLock){
System.out.println(Thread.currentThread().getName()+":rightleft: get right");
synchronized (leftLock){
System.out.println(Thread.currentThread().getName()+":rightleft: get left");
}
}
}
}
運行后輸出如下
aThread:leftRight:get left
bThread:rightleft: get right
可以通過jstack發(fā)現(xiàn)死鎖的痕跡
"bThread" prio=5 tid=0x00007fabb2001000 nid=0x5503 waiting for monitor entry [0x000000011d54b000] java.lang.Thread.State: BLOCKED (on object monitor) at main.lockTest.Test.rightleft(Test.java:52) - waiting to lock <0x00000007aaee5748> (a java.lang.Object) - locked <0x00000007aaee5758> (a java.lang.Object) at main.lockTest.Test$2.run(Test.java:30) at java.lang.Thread.run(Thread.java:745) Locked ownable synchronizers: - None "aThread" prio=5 tid=0x00007fabb2801000 nid=0x5303 waiting for monitor entry [0x000000011d448000] java.lang.Thread.State: BLOCKED (on object monitor) at main.lockTest.Test.leftRight(Test.java:43) - waiting to lock <0x00000007aaee5758> (a java.lang.Object) - locked <0x00000007aaee5748> (a java.lang.Object) at main.lockTest.Test$1.run(Test.java:19) at java.lang.Thread.run(Thread.java:745) Locked ownable synchronizers: - None
可以看到bThread持有鎖0x00000007aaee5758,同時等待0x00000007aaee5748,然而恰好aThread持有鎖0x00000007aaee5748并等待0x00000007aaee5758,從而形成了死鎖
線程饑餓死鎖
public class ExecutorLock {
private static ExecutorService single=Executors.newSingleThreadExecutor();
public static class AnotherCallable implements Callable<String>{
@Override public String call() throws Exception {
System.out.println("in AnotherCallable");
return "annother success";
}
}
public static class MyCallable implements Callable<String>{
@Override public String call() throws Exception {
System.out.println("in MyCallable");
Future<String> submit = single.submit(new AnotherCallable());
return "success:"+submit.get();
}
}
public static void main(String[] args) throws ExecutionException, InterruptedException {
MyCallable task = new MyCallable();
Future<String> submit = single.submit(task);
System.out.println(submit.get());
System.out.println("over");
single.shutdown();
}
}
執(zhí)行的輸出只有一行
in MyCallable
通過jstack觀察可以看到如下
"main" prio=5 tid=0x00007fab3f000000 nid=0x1303 waiting on condition [0x0000000107d63000] java.lang.Thread.State: WAITING (parking) at sun.misc.Unsafe.park(Native Method) - parking to wait for <0x00000007aaeed1d8> (a java.util.concurrent.FutureTask) at java.util.concurrent.locks.LockSupport.park(LockSupport.java:186) at java.util.concurrent.FutureTask.awaitDone(FutureTask.java:425) at java.util.concurrent.FutureTask.get(FutureTask.java:187) at main.lockTest.ExecutorLock.main(ExecutorLock.java:32) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:606) at com.intellij.rt.execution.application.AppMain.main(AppMain.java:140) Locked ownable synchronizers: - None .. "pool-1-thread-1" prio=5 tid=0x00007fab3f835800 nid=0x5303 waiting on condition [0x00000001199ee000] java.lang.Thread.State: WAITING (parking) at sun.misc.Unsafe.park(Native Method) - parking to wait for <0x00000007ab0f8698> (a java.util.concurrent.FutureTask) at java.util.concurrent.locks.LockSupport.park(LockSupport.java:186) at java.util.concurrent.FutureTask.awaitDone(FutureTask.java:425) at java.util.concurrent.FutureTask.get(FutureTask.java:187) at main.lockTest.ExecutorLock$MyCallable.call(ExecutorLock.java:26) at main.lockTest.ExecutorLock$MyCallable.call(ExecutorLock.java:20) at java.util.concurrent.FutureTask.run(FutureTask.java:262) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615) at java.lang.Thread.run(Thread.java:745) Locked ownable synchronizers: - <0x00000007aaeed258> (a java.util.concurrent.ThreadPoolExecutor$Worker)
主線程在等待一個FutureTask完成,而線程池中一個線程也在等待一個FutureTask完成。
從代碼實現(xiàn)可以看到,主線程往線程池中扔了一個任務A,任務A又往同一個線程池中扔了一個任務B,并等待B的完成,由于線程池中只有一個線程,這將導致B會被停留在阻塞隊列中,而A還得等待B的完成,這也就是互相等待導致了死鎖的反生
這種由于正在執(zhí)行的任務線程都在等待其它工作隊列中的任務而阻塞的現(xiàn)象稱為 線程饑餓死鎖
活鎖
并未產(chǎn)生線程阻塞,但是由于某種問題的存在,導致無法繼續(xù)執(zhí)行的情況。
1、消息重試。當某個消息處理失敗的時候,一直重試,但重試由于某種原因,比如消息格式不對,導致解析失敗,而它又被重試
這種時候一般是將不可修復的錯誤不要重試,或者是重試次數(shù)限定
2、相互協(xié)作的線程彼此響應從而修改自己狀態(tài),導致無法執(zhí)行下去。比如兩個很有禮貌的人在同一條路上相遇,彼此給對方讓路,但是又在同一條路上遇到了?;ハ嘀g反復的避讓下去
這種時候可以選擇一個隨機退讓,使得具備一定的隨機性
以上就是本文的全部內(nèi)容,希望對大家的學習有所幫助,也希望大家多多支持腳本之家。
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