java面試-死鎖產生、定位分析和修復

死鎖發生：兩個或多個線程之間，互相持有對方需要的鎖，而永久處於阻塞狀態

一、手寫死鎖代碼：

public class DeadLockSample extends Thread {
    private String first;
    private String second;

    public DeadLockSample(String name, String first, String second) {
        super(name);
        this.first = first;
        this.second = second;
    }


    @Override
    public void run() {
        synchronized (first) {
            System.out.println(this.getName() + " obtained:" + first);
            try {
                TimeUnit.SECONDS.sleep(1);
                synchronized (second){
                    System.out.println(this.getName() + " obtained:" + second);
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }


    public static void main(String[] args) throws InterruptedException{
        String lockA = "lockA";
        String lockB = "lockB";
        DeadLockSample deadLockSample1 = new DeadLockSample("thread1",lockA,lockB);
        DeadLockSample deadLockSample2 = new DeadLockSample("thread2",lockB,lockA);
        deadLockSample1.start();
        deadLockSample2.start();
        deadLockSample1.join();
        deadLockSample2.join();

    }
}

 

二、死鎖產生的四個條件：
互斥：共享資源X和Y只能被一個線程占用
占有且等待：線程T1已經獲取共享資源X，在等待共享資源Y的時候，不釋放共享資源X
不可搶占：其他線程不能強行搶占線程T1占有的資源
循環等待：線程T1等待線程T2占有的資源，線程T2等待線程T1占有的資源，這就是循環等待。

三、死鎖定位：

jps -l

jstack pid

Found one Java-level deadlock:
=============================
"thread2":
  waiting to lock monitor 0x00007fa34c016148 (object 0x00000007957fc7d0, a java.lang.String),
  which is held by "thread1"
"thread1":
  waiting to lock monitor 0x00007fa34c016358 (object 0x00000007957fc808, a java.lang.String),
  which is held by "thread2"

Java stack information for the threads listed above:
===================================================
"thread2":
        at com.example.demo.javaLession.lession18.DeadLockSample.run(DeadLockSample.java:41)
        - waiting to lock <0x00000007957fc7d0> (a java.lang.String)
        - locked <0x00000007957fc808> (a java.lang.String)
"thread1":
        at com.example.demo.javaLession.lession18.DeadLockSample.run(DeadLockSample.java:41)
        - waiting to lock <0x00000007957fc808> (a java.lang.String)
        - locked <0x00000007957fc7d0> (a java.lang.String)

Found 1 deadlock.

四、預防死鎖：

破壞占有且等待條件：保證一次申請所有的資源。

破壞不可搶占條件：synchronized無法做到，synchronized申請不到資源直接進入阻塞狀態。

java.util.concurrent Lock可以解決此問題

破壞循環等待條件：需要對資源進行排序，然后按序申請資源

五、修復死鎖

發生死鎖無法在線解決，必須重啟，修正程序本身的問題