3.10、活跃性、死锁、哲学家就餐、活锁、饥饿

本文主要是介绍3.10、活跃性、死锁、哲学家就餐、活锁、饥饿，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

死锁

有这样的情况：一个线程需要同时获得多把锁，这时就容易发生死锁
t1线程获得A对象锁，接下来想获取B对象的锁，t2线程获得B对象锁，接下来想要获取A对象锁

例：

		Object A = new Object();Object B = new Object();new Thread(() -> {synchronized (A) {log.debug("lock A");try {Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}synchronized (B) {log.debug("lock B");log.debug("do..");}}}, "t1").start();new Thread(() -> {synchronized (B) {log.debug("lock B");try {Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}synchronized (A) {log.debug("lock A");log.debug("do..");}}}, "t2").start();

输出

2022/03/06-18:20:57.484 [t1] c.Test1 - lock A
2022/03/06-18:20:57.484 [t2] c.Test1 - lock B

定位死锁

• 检测死锁可以使用jconsole工具，或使用jps定位进程id，在用jstack定位死锁

D:\self\IdeaProjects\juc-test\target\classes>jps
19764 Test1
14636 RemoteMavenServer36
14988
17052 Launcher
18636 Jps

D:\self\IdeaProjects\juc-test\target\classes>jstack 19764
2022-03-06 18:22:58
Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.162-b12 mixed mode):"DestroyJavaVM" #14 prio=5 os_prio=0 tid=0x0000000003343800 nid=0x210 waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLE"t2" #13 prio=5 os_prio=0 tid=0x000000001fb44000 nid=0x299c waiting for monitor entry [0x000000002031f000]java.lang.Thread.State: BLOCKED (on object monitor)at deadlock.Test1.lambda$main$1(Test1.java:39)- waiting to lock <0x000000076decb288> (a java.lang.Object)- locked <0x000000076decb298> (a java.lang.Object)at deadlock.Test1$$Lambda$2/245672235.run(Unknown Source)at java.lang.Thread.run(Thread.java:748)"t1" #12 prio=5 os_prio=0 tid=0x000000001fb69800 nid=0x160 waiting for monitor entry [0x000000002021f000]java.lang.Thread.State: BLOCKED (on object monitor)at deadlock.Test1.lambda$main$0(Test1.java:25)- waiting to lock <0x000000076decb298> (a java.lang.Object)- locked <0x000000076decb288> (a java.lang.Object)at deadlock.Test1$$Lambda$1/1321640594.run(Unknown Source)at java.lang.Thread.run(Thread.java:748)"Service Thread" #11 daemon prio=9 os_prio=0 tid=0x000000001ec78000 nid=0xe9c runnable [0x0000000000000000]java.lang.Thread.State: RUNNABLE"C1 CompilerThread3" #10 daemon prio=9 os_prio=2 tid=0x000000001ebe3800 nid=0x48d4 waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLE"C2 CompilerThread2" #9 daemon prio=9 os_prio=2 tid=0x000000001ebdf000 nid=0x4998 waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLE"C2 CompilerThread1" #8 daemon prio=9 os_prio=2 tid=0x000000001ebda000 nid=0x3eb0 waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLE"C2 CompilerThread0" #7 daemon prio=9 os_prio=2 tid=0x000000001ebd9800 nid=0x19e8 waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLE"Monitor Ctrl-Break" #6 daemon prio=5 os_prio=0 tid=0x000000001ebd6800 nid=0x4f58 runnable [0x000000001f26e000]java.lang.Thread.State: RUNNABLEat java.net.SocketInputStream.socketRead0(Native Method)at java.net.SocketInputStream.socketRead(SocketInputStream.java:116)at java.net.SocketInputStream.read(SocketInputStream.java:171)at java.net.SocketInputStream.read(SocketInputStream.java:141)at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:284)at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:326)at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:178)- locked <0x000000076d2c4d08> (a java.io.InputStreamReader)at java.io.InputStreamReader.read(InputStreamReader.java:184)at java.io.BufferedReader.fill(BufferedReader.java:161)at java.io.BufferedReader.readLine(BufferedReader.java:324)- locked <0x000000076d2c4d08> (a java.io.InputStreamReader)at java.io.BufferedReader.readLine(BufferedReader.java:389)at com.intellij.rt.execution.application.AppMainV2$1.run(AppMainV2.java:47)"Attach Listener" #5 daemon prio=5 os_prio=2 tid=0x000000001ea91000 nid=0x246c waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLE"Signal Dispatcher" #4 daemon prio=9 os_prio=2 tid=0x000000001ea90800 nid=0x4ba4 runnable [0x0000000000000000]java.lang.Thread.State: RUNNABLE"Finalizer" #3 daemon prio=8 os_prio=1 tid=0x000000001ea21800 nid=0x1528 in Object.wait() [0x000000001eeff000]java.lang.Thread.State: WAITING (on object monitor)at java.lang.Object.wait(Native Method)- waiting on <0x000000076d008ec0> (a java.lang.ref.ReferenceQueue$Lock)at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:143)- locked <0x000000076d008ec0> (a java.lang.ref.ReferenceQueue$Lock)at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:164)at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:212)"Reference Handler" #2 daemon prio=10 os_prio=2 tid=0x000000000343a000 nid=0x4ac0 in Object.wait() [0x000000001e9ff000]java.lang.Thread.State: WAITING (on object monitor)at java.lang.Object.wait(Native Method)- waiting on <0x000000076d006b68> (a java.lang.ref.Reference$Lock)at java.lang.Object.wait(Object.java:502)at java.lang.ref.Reference.tryHandlePending(Reference.java:191)- locked <0x000000076d006b68> (a java.lang.ref.Reference$Lock)at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)"VM Thread" os_prio=2 tid=0x000000001cb39000 nid=0x4fcc runnable"GC task thread#0 (ParallelGC)" os_prio=0 tid=0x0000000003359000 nid=0x4610 runnable"GC task thread#1 (ParallelGC)" os_prio=0 tid=0x000000000335a800 nid=0x1c4c runnable"GC task thread#2 (ParallelGC)" os_prio=0 tid=0x000000000335c800 nid=0x31d8 runnable"GC task thread#3 (ParallelGC)" os_prio=0 tid=0x000000000335e000 nid=0x4df0 runnable"GC task thread#4 (ParallelGC)" os_prio=0 tid=0x0000000003360000 nid=0x4410 runnable"GC task thread#5 (ParallelGC)" os_prio=0 tid=0x0000000003362800 nid=0x41d4 runnable"GC task thread#6 (ParallelGC)" os_prio=0 tid=0x0000000003365800 nid=0x3cd8 runnable"GC task thread#7 (ParallelGC)" os_prio=0 tid=0x0000000003366800 nid=0x4088 runnable"VM Periodic Task Thread" os_prio=2 tid=0x000000001ecea800 nid=0x4898 waiting on conditionJNI global references: 316Found one Java-level deadlock:
=============================
"t2":waiting to lock monitor 0x000000001cb40da8 (object 0x000000076decb288, a java.lang.Object),which is held by "t1"
"t1":waiting to lock monitor 0x000000001cb42d48 (object 0x000000076decb298, a java.lang.Object),which is held by "t2"Java stack information for the threads listed above:
===================================================
"t2":at deadlock.Test1.lambda$main$1(Test1.java:39)- waiting to lock <0x000000076decb288> (a java.lang.Object)- locked <0x000000076decb298> (a java.lang.Object)at deadlock.Test1$$Lambda$2/245672235.run(Unknown Source)at java.lang.Thread.run(Thread.java:748)
"t1":at deadlock.Test1.lambda$main$0(Test1.java:25)- waiting to lock <0x000000076decb298> (a java.lang.Object)- locked <0x000000076decb288> (a java.lang.Object)at deadlock.Test1$$Lambda$1/1321640594.run(Unknown Source)at java.lang.Thread.run(Thread.java:748)Found 1 deadlock.

• 避免死锁要注意加锁顺序
• 另外如果由于某个线程进入了死循环，导致其他线程一直等待，对于这种情况linux下可以通过top先定位到CPU占用高的Java进程，再利用top -Hp 进程id来定位是哪个线程，最后在用jstack排查

哲学家就餐问题

有五位哲学家，未做在圆桌旁

• 他们只做两件事情，思考和吃饭，思考一会吃口饭，吃完饭后接着思考
• 吃饭时要用两根筷子吃，桌上一共5根筷子，美味哲学家左右手各有一根筷子
• 如果筷子被身边的人拿着，自己就等待

筷子类

public class Chopstick {String name;public Chopstick(String name) {this.name = name;}@Overridepublic String toString() {return "Chopstick{" +"name='" + name + '\'' +'}';}
}

哲学家类

public class Philosoper extends Thread {Chopstick left;Chopstick right;public Philosoper(String name, Chopstick left, Chopstick right) {super(name);this.left = left;this.right = right;}public void eat() {try {log.debug("eating");Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}}@Overridepublic void run() {while (true) {synchronized (left) {synchronized (right) {eat();}}}}
}

测试

	public static void main(String[] args) {Chopstick c1 = new Chopstick("1");Chopstick c2 = new Chopstick("2");Chopstick c3 = new Chopstick("3");Chopstick c4 = new Chopstick("4");Chopstick c5 = new Chopstick("5");new Philosoper("苏格拉底", c1, c2).start();new Philosoper("柏拉图", c2, c3).start();new Philosoper("亚里士多德", c3, c4).start();new Philosoper("赫拉克勒斯", c4, c5).start();new Philosoper("阿基米德", c5, c1).start();}

输出

2022/03/06-21:53:30.380 [苏格拉底] c.Philosoper - eating
2022/03/06-21:53:30.380 [亚里士多德] c.Philosoper - eating
2022/03/06-21:53:31.386 [阿基米德] c.Philosoper - eating
// 卡住了

使用jconsole检查死锁

这种线程没有按预期结束，执行不下去的情况，归类为【活跃性】问题，除了死锁以外，还有活锁和饥饿者两种情况

活锁

活锁出现在两个线程互相改变对方的结束条件，最后谁也无法结束

static volatile int count = 10;public static void main(String[] args) {new Thread(() -> {while (count > 0) {try {Thread.sleep(200);} catch (InterruptedException e) {e.printStackTrace();}count--;log.debug("count:{}",count);}},"t1").start();new Thread(() -> {while (count < 20) {try {Thread.sleep(200);} catch (InterruptedException e) {e.printStackTrace();}count++;log.debug("count:{}",count);}},"t2").start();}

饥饿

很多教程中把饥饿定义为，一个线程优先级太低，是中得不到CPU调度执行，也不能够结束，饥饿的情况不易演示，讲读写锁会涉及饥饿问题

下面是一个线程饥饿的例子，先看看使用顺序加锁方式解决之前的死锁问题

顺序加锁的解决方案

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