synchronized锁升级那点事
synchronized的升级过程
引入JOL(Java Object Layout)来打印java对象头在内存中的字节码。
<dependency>
<groupId>org.openjdk.jol</groupId>
<artifactId>jol-core</artifactId>
<version>0.10</version>
</dependency>
对象创建时
对象创建时的状态有两种：
- 无锁
- 匿名偏向锁
关闭了偏向锁或者在偏向延迟内创建的对象锁的状态为无锁（01）。
Object object = new Object();
System.out.println(ClassLayout.parseInstance(object).toPrintable());
object对应的对象结构如下：
java.lang.Object object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
可以用虚拟机参数-XX:-UseBiasedLocking关闭偏向锁，JDK8默认开启。
可以用虚拟机参数-XX:BiasedLockingStartupDelay来调整偏向延迟的时间，默认4000ms，设置为0，表示不延迟。
偏向延迟时间后创建的对象锁的状态为匿名偏向锁（101）。
TimeUnit.SECONDS.sleep(5);
Object object = new Object();
System.out.println(ClassLayout.parseInstance(object).toPrintable());
object对应的对象结构如下：
java.lang.Object object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 05 00 00 00 (00000101 00000000 00000000 00000000) (5)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
为什么JVM默认开启偏向延迟？因为JVM虚拟机自己有一些默认启动的线程，里面有大量的同步代码，这些同步代码启动时就知道肯定会有竞争，如果使用偏向锁，就会造成偏向锁不断的进行锁撤销和锁升级的操作，效率较低。
无锁升级为轻量级锁
测试代码如下：
Object object = new Object();
System.out.println(ClassLayout.parseInstance(object).toPrintable());
synchronized (object) {
System.out.println(ClassLayout.parseInstance(object).toPrintable());
}
System.out.println(ClassLayout.parseInstance(object).toPrintable());
object对应的对象结构如下：
java.lang.Object object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
java.lang.Object object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 48 e1 58 02 (01001000 11100001 01011000 00000010) (39379272)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
java.lang.Object object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
object对象刚创建出来时由于还在偏向延迟内，所以状态为无锁（01），当使用synchronized进行同步时，无锁（01）就会升级为轻量级锁（00），当锁释放后，锁的状态就会变为无锁（01）。
匿名偏向升级为偏向锁
测试代码如下
TimeUnit.SECONDS.sleep(5); // 也可以不休眠，设置偏向延迟时间为0，-XX:BiasedLockingStartupDelay=0
Object object = new Object();
System.out.println(ClassLayout.parseInstance(object).toPrintable());
synchronized (object) {
System.out.println(ClassLayout.parseInstance(object).toPrintable());
}
System.out.println(ClassLayout.parseInstance(object).toPrintable());
object对应的对象结构如下：
java.lang.Object object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 05 00 00 00 (00000101 00000000 00000000 00000000) (5)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
java.lang.Object object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 05 98 47 02 (00000101 10011000 01000111 00000010) (38246405)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
java.lang.Object object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 05 98 47 02 (00000101 10011000 01000111 00000010) (38246405)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
当开启了偏向锁，到达偏向延迟时间后创建出来的对象锁的状态为匿名偏向锁（001），当使用synchronized进行同步时，锁会偏向于第一次持有锁的线程，锁状态对应的标志位不变，但是对象头中会存储持有锁线程的ID，当锁释放后，锁的状态不变，对象头的结构也不会变，便于下次曾经持有锁的再次来获取锁，只需要判断线程ID是否是自己，无需再次偏向，效率高。
匿名偏向锁变为无锁
测试代码：
TimeUnit.SECONDS.sleep(5); // 也可以不休眠，设置偏向延迟时间为0，-XX:BiasedLockingStartupDelay=0
Object object = new Object();
System.out.println(ClassLayout.parseInstance(object).toPrintable());
object.hashCode();
System.out.println(ClassLayout.parseInstance(object).toPrintable());
object对应的对象结构如下：
java.lang.Object object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 05 00 00 00 (00000101 00000000 00000000 00000000) (5)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
java.lang.Object object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 8f 3d 44 (00000001 10001111 00111101 01000100) (1144884993)
4 4 (object header) 61 00 00 00 (01100001 00000000 00000000 00000000) (97)
8 4 (object header) e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
当开启了偏向锁，到达偏向延迟时间后创建出来的对象锁的状态为匿名偏向锁（001），当计算对象的hashcode时，hashcode会覆盖偏向锁标志位的空间，所以锁的状态由匿名偏向锁（001）变为无锁（01）。
偏向锁升级为轻量级锁
轻量级锁也称为自旋锁，无锁，自适应锁。
偏向锁使用了一种等到竞争出现才释放锁的机制，所以当其他线程尝试竞争偏向锁时，持有偏向锁的线程才会释放锁。
线程在执行同步块之前，JVM会先在当前线程的栈桢中创建用于存储锁记录的空间，并将对象头中的Mark Word复制到锁记录中，官方称为Displaced Mark Word。然后线程尝试使用CAS将对象头中的Mark Word替换为指向锁记录的指针。如果成功，当前线程获得锁，如果失败，表示其他线程竞争锁，当前线程便尝试使用自旋来获取锁。
当一个对象的锁状态为偏向锁时，另外一个线程来竞争锁，不管持有锁的线程的同步块有没有执行完，这把锁都会进行撤销。偏向锁的撤销，需要等待全局安全点（在这个时间点上没有正在执行的字节码）。它会首先暂停拥有偏向锁的线程（Stop the World），然后检查持有偏向锁的线程是否活着，如果线程不处于活动状态，则将对象头设置成无锁状态；如果线程仍然活着，先将对象恢复成无锁状态，再将无锁升级为轻量级锁（遍历偏向锁线程的栈帧，如果里面还有LockRecord记录，说明还持有锁，那么让持有偏向锁的线程持有轻量级锁，否则让另一个线程持有轻量级锁），最后唤醒暂停的线程。
也就是偏向锁升级为轻量级锁的过程中，会涉及到偏向锁的撤销，锁的撤销会消耗系统资源，所以，在锁竞争特别激烈的时候，用偏向锁未必效率高，还不如直接使用轻量级锁。这也是为什么jvm偏向锁的默认设置要延迟4s，因为在jvm启动过程中会开启多个线程，有大量的锁竞争的操作。
偏向锁升级为轻量级锁的代码演示：
TimeUnit.SECONDS.sleep(5); // 休眠5s，保证偏向锁开启
Object object = new Object();
Thread t = new Thread(() -> {
synchronized (object) {
}
});
t.start();
t.join(); // 等待t1执行完
System.out.println(ClassLayout.parseInstance(object).toPrintable());
synchronized (object) {
System.out.println(ClassLayout.parseInstance(object).toPrintable());
}
System.out.println(ClassLayout.parseInstance(object).toPrintable());
运行结果如下：
java.lang.Object object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 05 b8 8f 1d (00000101 10111000 10001111 00011101) (495958021)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
java.lang.Object object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) e0 e7 90 02 (11100000 11100111 10010000 00000010) (43050976)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
java.lang.Object object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
从上面的运行结果可以发现对象头的锁状态由偏向锁（101）变成了轻量级锁（00），最后锁释放后变成了无锁（01）。
偏向锁升级为重量级锁
测试代码如下：
TimeUnit.SECONDS.sleep(5); // 休眠5s，保证偏向锁开启
Object object = new Object();
System.out.println(ClassLayout.parseInstance(object).toPrintable()); // 101
Thread t = new Thread(() -> {
synchronized (object) {
System.out.println(ClassLayout.parseInstance(object).toPrintable()); // 101
try {
object.wait(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(ClassLayout.parseInstance(object).toPrintable()); // 10
}
});
t.start();
t.join();
TimeUnit.SECONDS.sleep(3); // 等待锁释放
System.out.println(ClassLayout.parseInstance(object).toPrintable()); // 01
运行结果如下：
java.lang.Object object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 05 00 00 00 (00000101 00000000 00000000 00000000) (5)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
java.lang.Object object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 05 20 a2 1e (00000101 00100000 10100010 00011110) (513941509)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
java.lang.Object object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 1a b2 06 1c (00011010 10110010 00000110 00011100) (470200858)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
java.lang.Object object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
总结，当调用对象的wait()方法锁的状态将会直接从偏向锁（101）直接进入重量级锁（10）。
轻量级锁升级为重量级锁
在jdk1.6以前，轻量级锁自旋超过10次或者自旋的线程数超过CPU核数的一半，就会升级为重量级锁，如果太多线程自旋，自旋就是个CAS获取锁的死循环，会导致CPU消耗过大，不如升级为重量级锁，进入等待队列（不消耗CPU），轻量级锁可以使用-XX:PreBlockSpin来开启。JDK6中变为默认开启，并且引入了自适应的自旋锁（适应性自旋锁）。
自适应自旋锁意味着自旋的时间（次数）不再固定，而是由前一次在同一个锁上的自旋时间及锁的拥有者的状态来决定。如果在同一个锁对象上，自旋等待刚刚成功获得过锁，并且持有锁的线程正在运行中，那么虚拟机就会认为这次自旋也是很有可能再次成功，进而它将允许自旋等待持续相对更长的时间。如果对于某个锁，自旋很少成功获得过，那在以后尝试获取这个锁时将可能省略掉自旋过程，直接阻塞线程，避免浪费处理器资源。
升级重量级需要向操作系统申请资源，CPU从用户态切换会内核态用，线程挂起，进入等待队列，等待操作系统的调度，然后再映射回用户空间。
测试代码如下：
Object object = new Object();
Thread t1 = new Thread(() -> {
synchronized (object) {
System.out.println("thread name: " + Thread.currentThread().getName());
System.out.println(ClassLayout.parseInstance(object).toPrintable()); // 00
try {
TimeUnit.SECONDS.sleep(5);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}, "t1");
Thread t2 = new Thread(() -> {
try {
TimeUnit.SECONDS.sleep(2); // 等待t1获取到锁，造成资源竞争
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (object) {
System.out.println("thread name: " + Thread.currentThread().getName());
System.out.println(ClassLayout.parseInstance(object).toPrintable()); // 10
}
}, "t2");
t1.start();
t2.start();
t1.join();
t2.join();
TimeUnit.SECONDS.sleep(10); // 休眠一会，锁的释放没那么快
System.out.println(ClassLayout.parseInstance(object).toPrintable()); // 01
运行结果如下：
thread name: t1
java.lang.Object object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 08 ef ea 1e (00001000 11101111 11101010 00011110) (518713096)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
thread name: t2
java.lang.Object object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 5a b2 f7 1b (01011010 10110010 11110111 00011011) (469217882)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
java.lang.Object object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
当t1持有轻量级锁的过程中，t2过来获取锁，造成资源的竞争，就会使得轻量级锁（00）膨胀为重量级锁（10），重量级锁释放后，锁的状态变回无锁。