锁的粒度减小，并发量也会增大，当然也会随之而来一些问题

示例

假如有一个大房子有两个功能：睡觉和学习，互不相干，t1线程要学习，t2线程要睡觉，如果都用一个房子的话，并发度很低，解决方法就是用多个房子

就用一个房子

public class Test01 {
    public static void main(String[] args) {
        BigRoom bigRoom = new BigRoom();
        new Thread(() -> {
            bigRoom.study();
        }, "t1").start();
        new Thread(() -> {
            bigRoom.sleep();
        }, "t2").start();
    }
}

@Slf4j(topic = "ch.BigRoom")
class BigRoom {
    public void sleep() {
        synchronized (this) {
            log.debug("sleeping 2 hour");
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public void study() {
        synchronized (this) {
            log.debug("study 1 hour");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

将房子划分为两个锁

public class Test02 {
    public static void main(String[] args) {
        BigRoom02 bigRoom02 = new BigRoom02();
        new Thread(() -> {
            bigRoom02.study();
        }, "t1").start();
        new Thread(() -> {
            bigRoom02.sleep();
        }, "t2").start();
    }
}

//两个方法不公用一个锁了
@Slf4j(topic = "ch.BigRoom02")
class BigRoom02 {
    private final Object studyRoom = new Object();
    private final Object bedRoom = new Object();

    public void sleep() {
        synchronized (bedRoom) {
            log.debug("睡觉2s");
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public void study() {
        synchronized (studyRoom) {
            log.debug("学习1s");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

锁粒度变小好处是可以增加并发度，坏处就是容易发生死锁

活跃性

死锁

如果存在这样的情况，一个线程需要同时获取多把锁，这时就容易发生死锁
假如：t1线程获取a对象锁，接下来想获取b对象锁，t2线程获取了b对象的锁，接下来想获取a对象的锁，这时候就可能会出现死锁
代码：

@Slf4j(topic = "ch.SiSuoTest01")
public class SiSuoTest01 {
    public static void main(String[] args) {
        Object a = new Object();
        Object b = new Object();
        new Thread(() -> {
            synchronized (a) {
                log.debug("已经获得a，想要获得b");
                synchronized (b) {
                    log.debug("已经获得b");
                }
            }
            log.debug("结束");
        }, "t1").start();
        new Thread(() -> {
            synchronized (b) {
                log.debug("已经获得b，想要获得a");
                synchronized (a) {
                    log.debug("已经获得a");
                }
            }
            log.debug("结束");
        }, "t2").start();
    }
}

定位死锁

检测死锁可以用jconslole工具，或者使用jps定位进程id，然后再用jstack定位死锁

D:\JAVAPROJECT\JUCdemo>jps
23152 TestWaitNotify04
20724
14988 SiSuoTest01
18396 Launcher
6092 Jps
//这里要查看的是SiSuoTest01的信息
D:\JAVAPROJECT\JUCdemo>jstack 14988

找到死锁信息

Found one Java-level deadlock:
=============================
"t2":
  waiting to lock monitor 0x000000001c450e88 (object 0x000000076d09e298, a java.lang.Object),
  which is held by "t1"
"t1":
  waiting to lock monitor 0x000000001f434938 (object 0x000000076d09e2a8, a java.lang.Object),
  which is held by "t2"

Java stack information for the threads listed above:
===================================================
"t2":
        at Cha01Thread.siSuo.SiSuoTest01.lambda$main$1(SiSuoTest01.java:23)
        - waiting to lock <0x000000076d09e298> (a java.lang.Object)
        - locked <0x000000076d09e2a8> (a java.lang.Object)
        at Cha01Thread.siSuo.SiSuoTest01$$Lambda$2/245672235.run(Unknown Source)
        at java.lang.Thread.run(Thread.java:748)
"t1":
        at Cha01Thread.siSuo.SiSuoTest01.lambda$main$0(SiSuoTest01.java:14)
        - waiting to lock <0x000000076d09e2a8> (a java.lang.Object)
        - locked <0x000000076d09e298> (a java.lang.Object)
        at Cha01Thread.siSuo.SiSuoTest01$$Lambda$1/1321640594.run(Unknown Source)
        at java.lang.Thread.run(Thread.java:748)

Found 1 deadlock.

避免死锁需要注意加锁顺序
如果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定位是哪个线程，最后用jstack进行排查