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:selfIdeaProjectsjuc-testtargetclasses>jps 19764 Test1 14636 RemoteMavenServer36 14988 17052 Launcher 18636 Jps
D:selfIdeaProjectsjuc-testtargetclasses>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: RUNNABLE
at 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$ReferenceHandler.run(Reference.java:153)
"VM Thread" os_prio=2 tid=0x000000001cb39000 nid=0x4fcc runnable
"GC task thread#0 (ParallelGC)" os_prio=0 tid=0x0000000003359000 nid=0x4610 runnable
"GC task thread#1 (ParallelGC)" os_prio=0 tid=0x000000000335a800 nid=0x1c4c runnable
"GC task thread#2 (ParallelGC)" os_prio=0 tid=0x000000000335c800 nid=0x31d8 runnable
"GC task thread#3 (ParallelGC)" os_prio=0 tid=0x000000000335e000 nid=0x4df0 runnable
"GC task thread#4 (ParallelGC)" os_prio=0 tid=0x0000000003360000 nid=0x4410 runnable
"GC task thread#5 (ParallelGC)" os_prio=0 tid=0x0000000003362800 nid=0x41d4 runnable
"GC task thread#6 (ParallelGC)" os_prio=0 tid=0x0000000003365800 nid=0x3cd8 runnable
"GC task thread#7 (ParallelGC)" os_prio=0 tid=0x0000000003366800 nid=0x4088 runnable
"VM Periodic Task Thread" os_prio=2 tid=0x000000001ecea800 nid=0x4898 waiting on condition
JNI global references: 316
Found one Java-level deadlock:
=============================
"t2":
waiting to lock monitor 0x000000001cb40da8 (object 0x000000076decb288, a java.lang.Object),
which is held by "t1"
"t1":
waiting to lock monitor 0x000000001cb42d48 (object 0x000000076decb298, a java.lang.Object),
which is held by "t2"
Java stack information for the threads listed above:
===================================================
"t2":
at deadlock.Test1.lambda$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;
}
@Override
public 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();
}
}
@Override
public 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
