摘要：比如兩個(gè)很有禮貌的人在同一條路上相遇，彼此給對(duì)方讓路，但是又在同一條路上遇到了。互相之間反復(fù)的避讓下去這種時(shí)候可以選擇一個(gè)隨機(jī)退讓，使得具備一定的隨機(jī)性

順序死鎖：過度加鎖，導(dǎo)致由于執(zhí)行順序的原因，互相持有對(duì)方正在等待的鎖

資源死鎖：多個(gè)線程在相同的資源上發(fā)生等待

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(yùn)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(shí)等待0x00000007aaee5748,然而恰好aThread持有鎖0x00000007aaee5748并等待0x00000007aaee5758,從而形成了死鎖

public class ExecutorLock {
    private static ExecutorService single=Executors.newSingleThreadExecutor();
    public static class AnotherCallable implements Callable{

        @Override        public String call() throws Exception {
            System.out.println("in AnotherCallable");
            return "annother success";
        }
    }


    public static class MyCallable implements Callable{

        @Override        public String call() throws Exception {
            System.out.println("in MyCallable");
            Future submit = single.submit(new AnotherCallable());
            return "success:"+submit.get();
        }
    }
    public static void main(String[] args) throws ExecutionException, InterruptedException {
        MyCallable task = new MyCallable();
        Future 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(gè)FutureTask完成，而線程池中一個(gè)線程也在等待一個(gè)FutureTask完成。
從代碼實(shí)現(xiàn)可以看到，主線程往線程池中扔了一個(gè)任務(wù)A，任務(wù)A又往同一個(gè)線程池中扔了一個(gè)任務(wù)B，并等待B的完成，由于線程池中只有一個(gè)線程，這將導(dǎo)致B會(huì)被停留在阻塞隊(duì)列中，而A還得等待B的完成，這也就是互相等待導(dǎo)致了死鎖的反生

這種由于正在執(zhí)行的任務(wù)線程都在等待其它工作隊(duì)列中的任務(wù)而阻塞的現(xiàn)象稱為 線程饑餓死鎖

并未產(chǎn)生線程阻塞，但是由于某種問題的存在，導(dǎo)致無法繼續(xù)執(zhí)行的情況。

消息重試。當(dāng)某個(gè)消息處理失敗的時(shí)候，一直重試，但重試由于某種原因，比如消息格式不對(duì)，導(dǎo)致解析失敗，而它又被重試

這種時(shí)候一般是將不可修復(fù)的錯(cuò)誤不要重試，或者是重試次數(shù)限定

相互協(xié)作的線程彼此響應(yīng)從而修改自己狀態(tài)，導(dǎo)致無法執(zhí)行下去。比如兩個(gè)很有禮貌的人在同一條路上相遇，彼此給對(duì)方讓路，但是又在同一條路上遇到了。互相之間反復(fù)的避讓下去

這種時(shí)候可以選擇一個(gè)隨機(jī)退讓，使得具備一定的隨機(jī)性