关于 synchronized 锁优化
JAVA乐园
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2021-01-18 21:40
众所周知，让开发者简单轻松的编写保证线程安全的代码，一直是现代编程语言所最求的，Java 也不例外。Java 语言引入的 synchronized 关键字，无不彰显它在此方面的勃勃雄心。但理想丰满现实骨感，早期的 Java 版本里，对于此关键字的实现太过厚重，导致线程同步的性能远不如预期。Java HotSpot™ VM 经过多个版本的迭代，利用锁膨胀思想，尽量延迟使用重量级锁的手段来提升 synchronized 原语的性能。
对象存储
对象结构
对象
对象在内存中的结构 （64位 JVM） 对象头 object header 成员变量 object field 对齐/填充（可选） alignment/padding gap (optional) 对象数组
对象数组在内存中的结构 （64位 JVM） 对象头 object header 数组元素字节序列 elements bytes 对齐/填充（可选） alignment/padding gap (optional)
由于对象在内存以 8 字节 为最小单位，单个对象占用内存字节不是 8 的倍数时，需要增加留空字节凑成倍数，此时留空的字节被称为 对象对齐（object alignment）。单个对象内部的成员变量所占字节不满 4 的倍数时，也需增加留空字节凑成倍数，此时的留空字节被称为 填充间隙（padding gap）。
对象头结构
对象
对象的 Object Hearder 结构 （64位 JVM） Mark Word （64 位） Klass Word（压缩 32位，不压缩 64位）1 对齐/填充（可选） alignment/padding gap (optional) 对象数组
对象数组的 Object Hearder 结构 （64位 JVM） Mark Word （64 位） Klass Word（压缩 32 位，不压缩 64 位） 数组长度（32 位） 对齐/填充（可选） alignment/padding gap (optional)
Mark Word 结构
Mark Word，为运行时对象的标记字，字在 32 位系统中占用 32 bit，64 位操作系统占用 64 bit。其记录着对象运行时的数据，包括 identity_hashcode、GC 分代年龄、锁状态 等信息。以下引用自 JDK8 HotSpot 源码 markOop.hpp 中，关于 Mark Word 结构信息的描述：
// 32 bits:
// --------
// hash:25 ------------>| age:4 biased_lock:1 lock:2 (normal object)
// JavaThread*:23 epoch:2 age:4 biased_lock:1 lock:2 (biased object)
// size:32 ------------------------------------------>| (CMS free block)
// PromotedObject*:29 ---------->| promo_bits:3 ----->| (CMS promoted object)
//
// 64 bits:
// --------
// unused:25 hash:31 -->| unused:1 age:4 biased_lock:1 lock:2 (normal object)
// JavaThread*:54 epoch:2 unused:1 age:4 biased_lock:1 lock:2 (biased object)
// PromotedObject*:61 --------------------->| promo_bits:3 ----->| (CMS promoted object)
// size:64 ----------------------------------------------------->| (CMS free block)
//
// unused:25 hash:31 -->| cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && normal object)
// JavaThread*:54 epoch:2 cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && biased object)
// narrowOop:32 unused:24 cms_free:1 unused:4 promo_bits:3 ----->| (COOPs && CMS promoted object)
// unused:21 size:35 -->| cms_free:1 unused:7 ------------------>| (COOPs && CMS free block)
为了节省内存空间，64 位的 JVM 采取压缩普通对象指针 (COOPs，即 Compressed Ordinary Object Pointer) 2 技术，把对象指针由原来的 64 bit 压缩成 32 bit，进而省了一半的内存空间。
我们着重关注 64 位 JVM 的锁的几种状态，也就是通过上面的 biased_lock、lock 两个标志位的排列组合得到：
| biased_lock | lock | 状态 |
|---|---|---|
| 0 | 01 | 无锁 normal object |
| 1 | 01 | 偏向锁 biased object |
| 0 | 00 | 轻量级锁 lightweight/thin object |
| 0 | 10 | 重量级锁 heavyweight/fat object |
| 0 | 11 | GC 标记 |
锁的种类
根据 Mark Word 中的锁状态，我们分别来介绍下。
重量级锁
利用系统级别的互斥量（mutex）实现同步临界区，由于系统级调用，开销大，故称其位重量级锁。
在下一节关于锁的状态改变图中，会发现一个 重量级监视器指针，由于它覆盖（官方称为 Displaced）了原本的 Mark Word，故它所指向的是复杂的数据结构 ObjectMonitor 就包含有用来存储备份的 Mark Word 信息。除此之外，该数据结构还包含锁的等待列表等信息，详细可参考 Monitor Object 设计模式。
优点：持有锁时间长、竞争激烈时，其他等待的线程会让出 CPU 进入等待列表
缺点：系统级调用，开销大
自旋锁
利用循环的方式来实现线程等待（忙等），等待期间内不让出 CPU 执行时间、免去系统级线程切换开销。
优点：避免线程切换
缺点：等待期间占用 CPU 资源
轻量级锁
采用原子性的 CAS 进行加解锁操作，加锁失败时自旋等待，成功则将 Mark Word 覆盖成指向线程栈中的 Lock Record 指针，此记录中同样含有原 Mark Word 记录的备份信息。CAS 调用不需系统级别调用，故称为轻量级锁。
优点：无需系统级调用，性能好于重量级锁
缺点：每次加解锁都需要一次 CAS 操作，采用自旋忙等，在竞争激烈、持有锁时间长会加重 CPU 负荷，会转到重量级锁。
偏向锁
给当前锁标记所属线程，使得所属线程进入同步临界区不用做任何特殊处理，只是简单的使用 CAS 操作将所属线程 ID 记录到 Mark Word 中，同一线程再次加解锁时无需 CAS 操作。
优点：只需初始化时进行一次 CAS 操作，之后加解锁都无需 CAS 操作
缺点：只能单线程无竞争时使用，一旦有其他线程就必须撤销转到轻量级。
锁转态转移
在给新建的对象分配内存时，其对象头信息会按照下图所示的进行分配，同时随着线程的竞争发送锁状态的转化：
状态转步骤
具体锁转移的过程如下：
如果偏向锁机制是启用，那么新建的对象被初始化成匿名的偏向锁。之所以是匿名，是因为此时并没有具体偏向的线程 ID。
使用虚拟机参数
-XX:-UseBiasedLocking可以关闭偏向锁，默认是启用的。由于虚拟机参数
-XX:BiasedLockingStartupDelay默认 4 秒，偏向锁机制会延迟 4 秒生效，测试时可以将其设置为 0 秒，防止偏向锁设置不生效。调用对象默认的
hashCode()、System.identityHashCode(obj)方法会使偏向锁膨胀为轻量级锁 3。原因为生成的identity_hashcode需要被记录到 Mark Word 中，而偏向锁结构没有设计存储备份 Mark Word 的指针，类似轻量级锁的lock record pointer、重量级锁的heavyweight monitor pointer。当有单个线程尝试获取该对象锁时，将把此线程的 ID 写入 Mark Word，完成加锁操作。
当此单线程解锁时，一直不存在其他线程来竞争时，此时重偏向 (rebias) 匿名偏向锁，即无线程 ID 的偏向锁。
一旦有其他线程时（不管有无竞争），就撤销偏向 (revoke bias) 转为轻量级锁。
其他线程 CAS 操作将自己的线程号 ID 放入 Mark Word 会失败，此线程会执行撤销偏向，在原偏向锁持有线程到达安全点后暂停原线程，检查原线程是否还持有锁。如果已释放锁，将锁状态修改为普通无锁状态；如果未释放锁，拷贝 Mark Word 到原偏向锁线程的锁记录中，修改锁状态标志位为轻量级，把指向原偏向锁线程的锁记录的指针存入 Mark Word 中，唤醒原持有偏向锁线程。
原持有偏向锁线程继续从安全点之后运行，解锁时判断对象头的锁记录指针是否指向当前线程锁记录、且锁记录的备份 Mark Word 与现有对象头里的 Mark Word 一致，如果都一致说明没有其他线程等待此锁，如果不一致说明有其他线程等待，释放锁之后需要换线挂起的那些线程。
轻量级锁状态时，如果竞争激烈（等待线程多）、原线程持锁时间长（即其他线程自旋次数多、等待时间长）就会膨胀为重量级锁。
轻量级、重量级锁解锁后转为普通无锁状态，即后三位为
001。当然，在重量级锁状态时，如果竞争转为不激烈时，锁会降级为轻量级状态。
状态转移验证源代码
代码依赖
代码结构
project
└── src
│ └── test
│ │ └── java
│ │ │ └── org
│ │ │ │ └── reion
│ │ │ │ │ └── LockTest.java
│ └── main
│ │ └── resources
│ │ └── java
│ │ │ └── org
│ │ │ │ └── reion
│ │ │ │ │ └── Student.java
代码清单
Student.java
package org.reion;
/**
* 学生
*/
public class Student {
// 名
String firstName;
// 姓
String lastName;
}LockTest.java
package org.reion;
import org.junit.Before;
import org.junit.Test;
import java.util.Random;
import org.openjdk.jol.info.ClassLayout;
import static java.lang.System.out;
public class OopTest {
public static final String LINE_SEPARATOR = "\n<<============= %s ==============>>";
Student stu1;
@Before
public void before() {
stu1 = new Student();
}
// 具体测试方法在下面单个列出，此处省略
}
偏向锁
测试方法
/**
* 该方法验证对象初始化后，由单个线程持有时的偏向锁形态。
*/
@Test
public void testBiasedLock() throws InterruptedException {
// 确保代码运行时，已经启用偏向锁机制，有两种方式：
// ① 偏向锁机制默认延迟 4 秒启动，故休眠 5 秒
Thread.sleep(5000);
// stu1 之所以重新赋值，因为 before 里旧对象由于偏向锁延迟启动机制，生成不可偏向的对象
stu1 = new Student();
// ② 添加虚拟机参数，把默认延迟修改为 0 秒，虚拟机启动立马启用偏向锁机制
// -XX:BiasedLockingStartupDelay=0
//【未加锁，可偏向】
out.println(String.format(LINE_SEPARATOR, "未加锁，可偏向"));
out.println(ClassLayout.parseInstance(stu1).toPrintable());
//【偏向锁】
synchronized (stu1) {
out.println(String.format(LINE_SEPARATOR, "偏向锁"));
out.println(ClassLayout.parseInstance(stu1).toPrintable());
}
//【未加锁，可偏向】
out.println(String.format(LINE_SEPARATOR, "未加锁，可偏向"));
out.println(ClassLayout.parseInstance(stu1).toPrintable());
}输出结果
<<============= 未加锁，可偏向 ==============>>
# WARNING: Unable to attach Serviceability Agent. You can try again with escalated privileges. Two options: a) use -Djol.tryWithSudo=true to try with sudo; b) echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope
org.reion.Student 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) b4 02 01 f8 (10110100 00000010 00000001 11111000) (-134151500)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
<<============= 偏向锁 ==============>>
org.reion.Student object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 05 a8 00 b9 (00000101 10101000 00000000 10111001) (-1191139323)
4 4 (object header) e1 7f 00 00 (11100001 01111111 00000000 00000000) (32737)
8 4 (object header) b4 02 01 f8 (10110100 00000010 00000001 11111000) (-134151500)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
<<============= 未加锁，可偏向 ==============>>
org.reion.Student object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 05 a8 00 b9 (00000101 10101000 00000000 10111001) (-1191139323)
4 4 (object header) e1 7f 00 00 (11100001 01111111 00000000 00000000) (32737)
8 4 (object header) b4 02 01 f8 (10110100 00000010 00000001 11111000) (-134151500)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
注意： 由于 JVM 为 小端法（Little Endian） 故低字节在前，高字节在后。
因此：
打印结果中 object header 中 64 位 Mark Word 字节序列应该为
00 00 7f e1 b9 00 a8 05
轻量级锁
测试方法
/**
* 该方法验证调用默认 hashCode 或 System.identityHashCode 时，变为轻量级锁的形态
*/
@Test
public void testThinLock() {
// 调用 System.identityHashCode(stu1) 使对象 stu1 禁用偏向锁
// TIPS：
// 让对象能够利用上偏向锁机制，应该重载对象的 hashCode 方法，
// 避免 identityHashCode 方法调用，直接膨胀为轻量级锁
out.println(String.format(LINE_SEPARATOR, "System.identityHashCode 方法调用"));
out.println(Integer.toHexString(stu1.hashCode()));
//【未加锁，不可偏向】
out.println(String.format(LINE_SEPARATOR, "未加锁，不可偏向"));
out.println(ClassLayout.parseInstance(stu1).toPrintable());
//【轻量级锁】
synchronized (stu1) {
out.println(String.format(LINE_SEPARATOR, "轻量锁"));
out.println(ClassLayout.parseInstance(stu1).toPrintable());
}
//【未加锁，不可偏向】
out.println(String.format(LINE_SEPARATOR, "未加锁，不可偏向"));
out.println(ClassLayout.parseInstance(stu1).toPrintable());
}输出结果
<<============= System.identityHashCode 方法调用 ==============>>
6aa8ceb6
<<============= 未加锁，不可偏向 ==============>>
# WARNING: Unable to attach Serviceability Agent. You can try again with escalated privileges. Two options: a) use -Djol.tryWithSudo=true to try with sudo; b) echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope
org.reion.Student object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 b6 ce a8 (00000001 10110110 11001110 10101000) (-1462847999)
4 4 (object header) 6a 00 00 00 (01101010 00000000 00000000 00000000) (106)
8 4 (object header) b4 02 01 f8 (10110100 00000010 00000001 11111000) (-134151500)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
<<============= 轻量锁 ==============>>
org.reion.Student object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 88 18 94 01 (10001000 00011000 10010100 00000001) (26482824)
4 4 (object header) 00 70 00 00 (00000000 01110000 00000000 00000000) (28672)
8 4 (object header) b4 02 01 f8 (10110100 00000010 00000001 11111000) (-134151500)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
<<============= 未加锁，不可偏向 ==============>>
org.reion.Student object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 b6 ce a8 (00000001 10110110 11001110 10101000) (-1462847999)
4 4 (object header) 6a 00 00 00 (01101010 00000000 00000000 00000000) (106)
8 4 (object header) b4 02 01 f8 (10110100 00000010 00000001 11111000) (-134151500)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
偏向锁 -> 轻量级锁
测试方法
/**
* 该方法验证第二个线程出现时（此时第二线程已经在第一个线程解锁后才执行，故无竞争），会撤销偏向，变为轻量级锁的形态。
*/
@Test
public void testBias2ThinLock() throws InterruptedException {
// 调用偏向锁方法，生成可偏向的 stu1 对象
testBiasedLock();
// 生成新线程，申请对象 stu1 锁
out.println("当前线程：" + Thread.currentThread());
Runnable runnable = () -> {
out.println("\n进入线程：" + Thread.currentThread());
out.println(String.format(LINE_SEPARATOR, "未请求锁，可偏向"));
out.println(ClassLayout.parseInstance(stu1).toPrintable());
//【偏向锁 -膨胀-> 轻量级锁】
synchronized (stu1) {
out.println(String.format(LINE_SEPARATOR, "已加锁，膨胀为轻量级锁"));
out.println(ClassLayout.parseInstance(stu1).toPrintable());
}
//【未加锁，不可偏向】
out.println(String.format(LINE_SEPARATOR, "未加锁，不可偏向"));
out.println(ClassLayout.parseInstance(stu1).toPrintable());
out.println("\n退出线程：" + Thread.currentThread());
};
Thread thread = new Thread(runnable);
thread.start();
thread.join();
}输出结果
当前线程：Thread[main,5,main]
进入线程：Thread[Thread-1,5,main]
<<============= 未请求锁，可偏向 ==============>>
org.reion.Student object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 05 d0 00 be (00000101 11010000 00000000 10111110) (-1107243003)
4 4 (object header) fb 7f 00 00 (11111011 01111111 00000000 00000000) (32763)
8 4 (object header) b4 02 01 f8 (10110100 00000010 00000001 11111000) (-134151500)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
<<============= 已加锁，膨胀为轻量级锁 ==============>>
org.reion.Student object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 08 c9 3e 06 (00001000 11001001 00111110 00000110) (104777992)
4 4 (object header) 00 70 00 00 (00000000 01110000 00000000 00000000) (28672)
8 4 (object header) b4 02 01 f8 (10110100 00000010 00000001 11111000) (-134151500)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
<<============= 未加锁，不可偏向 ==============>>
org.reion.Student 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) b4 02 01 f8 (10110100 00000010 00000001 11111000) (-134151500)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
轻量级锁维持
测试方法
/**
* 该方法验证低竞争度时，轻量级锁能一直维持此状态，从而提高运行效率
*/
@Test
public void testLowContentionThinLock() throws InterruptedException {
// 调用 testBias2ThinLock，使 stu1 经历 偏向锁 -> 轻量锁 过程
// 最终使 stu1 成为 未加锁不可偏
testBias2ThinLock();
Runnable runnable = () -> {
Thread current = Thread.currentThread();
out.println("\n进入线程：" + current);
//【轻量级锁】
for (int i = 0; i<2; i++) {
try {
// 400 毫秒内的随机休眠时间，模拟低对象锁竞争
Thread.sleep(new Random().nextInt(400));
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (stu1) {
out.println(String.format(LINE_SEPARATOR, current + " ROUND-" + i) + "\n" + ClassLayout.parseInstance(stu1).toPrintable());
}
}
out.println("\n退出线程：" + Thread.currentThread());
};
Thread t1 = new Thread(runnable);
Thread t2 = new Thread(runnable);
t1.start();
t2.start();
t1.join();
t2.join();
// 结束竞争后对象状态
out.println(String.format(LINE_SEPARATOR, Thread.currentThread()) + "\n" + ClassLayout.parseInstance(stu1).toPrintable());
}输出结果
进入线程：Thread[Thread-2,5,main]
进入线程：Thread[Thread-3,5,main]
<<============= Thread[Thread-3,5,main] ROUND-0 ==============>>
org.reion.Student object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) f8 98 1e 11 (11111000 10011000 00011110 00010001) (287217912)
4 4 (object header) 00 70 00 00 (00000000 01110000 00000000 00000000) (28672)
8 4 (object header) 83 02 01 f8 (10000011 00000010 00000001 11111000) (-134151549)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
<<============= Thread[Thread-3,5,main] ROUND-1 ==============>>
org.reion.Student object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) f8 98 1e 11 (11111000 10011000 00011110 00010001) (287217912)
4 4 (object header) 00 70 00 00 (00000000 01110000 00000000 00000000) (28672)
8 4 (object header) 83 02 01 f8 (10000011 00000010 00000001 11111000) (-134151549)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
退出线程：Thread[Thread-3,5,main]
<<============= Thread[Thread-2,5,main] ROUND-0 ==============>>
org.reion.Student object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) f8 68 0e 11 (11111000 01101000 00001110 00010001) (286157048)
4 4 (object header) 00 70 00 00 (00000000 01110000 00000000 00000000) (28672)
8 4 (object header) 83 02 01 f8 (10000011 00000010 00000001 11111000) (-134151549)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
<<============= Thread[Thread-2,5,main] ROUND-1 ==============>>
org.reion.Student object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) f8 68 0e 11 (11111000 01101000 00001110 00010001) (286157048)
4 4 (object header) 00 70 00 00 (00000000 01110000 00000000 00000000) (28672)
8 4 (object header) 83 02 01 f8 (10000011 00000010 00000001 11111000) (-134151549)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
退出线程：Thread[Thread-2,5,main]
<<============= Thread[main,5,main] ==============>>
org.reion.Student 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) 83 02 01 f8 (10000011 00000010 00000001 11111000) (-134151549)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
轻量级锁 -> 重量级锁 (高频型)
测试方法
/**
* 该方法验证竞争变激烈（请求锁频率快）后，锁由轻量级膨胀为重量级的过程。
*/
@Test
public void testHighContentionThin2FatLock() throws InterruptedException {
// 调用 testBias2ThinLock，使 stu1 经历 偏向锁 -> 轻量锁 过程
// 最终使 stu1 成为 未加锁不可偏
testBias2ThinLock();
Runnable runnable = () -> {
Thread current = Thread.currentThread();
out.println("\n进入线程：" + current);
//【轻量级锁】
for (int i = 0; i < 2; i++) {
try {
// 5 毫秒内的随机休眠时间，模拟高对象锁竞争
Thread.sleep(new Random().nextInt(5));
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (stu1) {
out.println(String.format(LINE_SEPARATOR, current + " ROUND-" + i) + "\n" + ClassLayout.parseInstance(stu1).toPrintable());
}
}
out.println("\n退出线程：" + Thread.currentThread());
};
Thread t1 = new Thread(runnable);
Thread t2 = new Thread(runnable);
t1.start();
t2.start();
t1.join();
t2.join();
// 结束竞争后对象状态
out.println(String.format(LINE_SEPARATOR, Thread.currentThread()) + "\n" + ClassLayout.parseInstance(stu1).toPrintable());
}输出结果
<<============= 未加锁，不可偏向 ==============>>
org.reion.Student 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) b4 02 01 f8 (10110100 00000010 00000001 11111000) (-134151500)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
退出线程：Thread[Thread-1,5,main]
进入线程：Thread[Thread-2,5,main]
进入线程：Thread[Thread-3,5,main]
<<============= Thread[Thread-2,5,main] ROUND-0 ==============>>
org.reion.Student object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) da d3 02 e4 (11011010 11010011 00000010 11100100) (-469576742)
4 4 (object header) 9a 7f 00 00 (10011010 01111111 00000000 00000000) (32666)
8 4 (object header) b4 02 01 f8 (10110100 00000010 00000001 11111000) (-134151500)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
<<============= Thread[Thread-3,5,main] ROUND-0 ==============>>
org.reion.Student object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) da d3 02 e4 (11011010 11010011 00000010 11100100) (-469576742)
4 4 (object header) 9a 7f 00 00 (10011010 01111111 00000000 00000000) (32666)
8 4 (object header) b4 02 01 f8 (10110100 00000010 00000001 11111000) (-134151500)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
<<============= Thread[Thread-2,5,main] ROUND-1 ==============>>
org.reion.Student object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) da d3 02 e4 (11011010 11010011 00000010 11100100) (-469576742)
4 4 (object header) 9a 7f 00 00 (10011010 01111111 00000000 00000000) (32666)
8 4 (object header) b4 02 01 f8 (10110100 00000010 00000001 11111000) (-134151500)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
退出线程：Thread[Thread-2,5,main]
<<============= Thread[Thread-3,5,main] ROUND-1 ==============>>
org.reion.Student object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) da d3 02 e4 (11011010 11010011 00000010 11100100) (-469576742)
4 4 (object header) 9a 7f 00 00 (10011010 01111111 00000000 00000000) (32666)
8 4 (object header) b4 02 01 f8 (10110100 00000010 00000001 11111000) (-134151500)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
退出线程：Thread[Thread-3,5,main]
<<============= Thread[main,5,main] ==============>>
org.reion.Student object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) da d3 02 e4 (11011010 11010011 00000010 11100100) (-469576742)
4 4 (object header) 9a 7f 00 00 (10011010 01111111 00000000 00000000) (32666)
8 4 (object header) b4 02 01 f8 (10110100 00000010 00000001 11111000) (-134151500)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
轻量级锁 -> 重量级锁 (长时型)
测试方法
/**
* 该方法验证持锁时间长（等待线程自旋过多），锁由轻量级膨胀为重量级的过程。
*/
@Test
public void testLongContentionThin2FatLock() throws InterruptedException {
// 调用 testBias2ThinLock，使 stu1 经历 偏向锁 -> 轻量锁 过程
// 最终使 stu1 成为 未加锁不可偏
testBias2ThinLock();
// 长期持有对象锁，在另一个线程开始竞争时，观察锁膨胀过程
Runnable runnable = () -> {
Thread current = Thread.currentThread();
out.println("\n进入线程：" + current);
//【轻量级锁】
synchronized (stu1) {
int counter = 0;
while (counter < 5) {
counter++;
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
out.println(String.format(LINE_SEPARATOR, current + " 锁内") + "\n" + ClassLayout.parseInstance(stu1).toPrintable());
}
}
out.println("\n退出线程：" + current);
};
// 睡眠 2.5 秒后，开始竞争锁
Runnable runnable2 = () -> {
Thread current = Thread.currentThread();
out.println("\n进入线程：" + current);
//【轻量级锁】
try {
out.println(String.format(LINE_SEPARATOR, current + "睡眠...") + "\n\n");
Thread.sleep(2500);
} catch (InterruptedException e) {
e.printStackTrace();
}
// 2.5 秒后参与竞争
out.println(String.format(LINE_SEPARATOR, current + "参与竞争...") + "\n\n");
synchronized (stu1) {
out.println(String.format(LINE_SEPARATOR, current) + "\n" + ClassLayout.parseInstance(stu1).toPrintable());
}
};
Thread t1 = new Thread(runnable);
Thread t2 = new Thread(runnable2);
t1.start();
t2.start();
t1.join();
t2.join();
}输出结果
进入线程：Thread[Thread-2,5,main]
进入线程：Thread[Thread-3,5,main]
<<============= Thread[Thread-3,5,main]睡眠... ==============>>
<<============= Thread[Thread-2,5,main] 锁内 ==============>>
org.reion.Student object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) f0 98 a9 0c (11110000 10011000 10101001 00001100) (212441328)
4 4 (object header) 00 70 00 00 (00000000 01110000 00000000 00000000) (28672)
8 4 (object header) b4 02 01 f8 (10110100 00000010 00000001 11111000) (-134151500)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
<<============= Thread[Thread-2,5,main] 锁内 ==============>>
org.reion.Student object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) f0 98 a9 0c (11110000 10011000 10101001 00001100) (212441328)
4 4 (object header) 00 70 00 00 (00000000 01110000 00000000 00000000) (28672)
8 4 (object header) b4 02 01 f8 (10110100 00000010 00000001 11111000) (-134151500)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
<<============= Thread[Thread-3,5,main]参与竞争... ==============>>
<<============= Thread[Thread-2,5,main] 锁内 ==============>>
org.reion.Student object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 8a 3c 01 5d (10001010 00111100 00000001 01011101) (1560362122)
4 4 (object header) d2 7f 00 00 (11010010 01111111 00000000 00000000) (32722)
8 4 (object header) b4 02 01 f8 (10110100 00000010 00000001 11111000) (-134151500)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
<<============= Thread[Thread-2,5,main] 锁内 ==============>>
org.reion.Student object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 8a 3c 01 5d (10001010 00111100 00000001 01011101) (1560362122)
4 4 (object header) d2 7f 00 00 (11010010 01111111 00000000 00000000) (32722)
8 4 (object header) b4 02 01 f8 (10110100 00000010 00000001 11111000) (-134151500)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
<<============= Thread[Thread-2,5,main] 锁内 ==============>>
org.reion.Student object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 8a 3c 01 5d (10001010 00111100 00000001 01011101) (1560362122)
4 4 (object header) d2 7f 00 00 (11010010 01111111 00000000 00000000) (32722)
8 4 (object header) b4 02 01 f8 (10110100 00000010 00000001 11111000) (-134151500)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
退出线程：Thread[Thread-2,5,main]
<<============= Thread[Thread-3,5,main] ==============>>
org.reion.Student object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 8a 3c 01 5d (10001010 00111100 00000001 01011101) (1560362122)
4 4 (object header) d2 7f 00 00 (11010010 01111111 00000000 00000000) (32722)
8 4 (object header) b4 02 01 f8 (10110100 00000010 00000001 11111000) (-134151500)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
轻量级锁 -> 重量级锁 -> 轻量级锁
测试方法
/**
* 该方法验证竞争由强转弱后，锁由轻量级膨胀为重量级后又降级为轻量级的过程。
*/
@Test
public void testHighContentionThin2FatThenDeflate2ThinLock() throws InterruptedException {
// 调用 testBias2ThinLock，使 stu1 经历 偏向锁 -> 轻量锁 过程
// 最终使 stu1 成为 未加锁不可偏
testBias2ThinLock();
Runnable runnable = () -> {
Thread current = Thread.currentThread();
out.println("\n进入线程：" + current);
// 随机产生 10 之内的循环追加数，模拟双线程中，一个线程完成后，剩下的线程单独执行时锁降级
int maxRound = new Random().nextInt(10);
//【轻量级锁】
for (int i = 0; i < 5 + maxRound; i++) {
try {
// 5 毫秒内的随机休眠时间，模拟高对象锁竞争
Thread.sleep(new Random().nextInt(5));
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (stu1) {
out.println(String.format(LINE_SEPARATOR, current + " ROUND-" + i) + "\n" + ClassLayout.parseInstance(stu1).toPrintable());
}
out.println(String.format(LINE_SEPARATOR, current + " ROUND-" + i) + "\n" + ClassLayout.parseInstance(stu1).toPrintable());
}
out.println("\n退出线程：" + Thread.currentThread());
};
Thread t1 = new Thread(runnable);
Thread t2 = new Thread(runnable);
t1.start();
t2.start();
t1.join();
t2.join();
// 结束竞争后对象状态
out.println(String.format(LINE_SEPARATOR, Thread.currentThread()) + "\n" + ClassLayout.parseInstance(stu1).toPrintable());
}输出结果
进入线程：Thread[Thread-2,5,main]
进入线程：Thread[Thread-3,5,main]
<<============= Thread[Thread-2,5,main] ROUND-0 ==============>>
org.reion.Student object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) f0 a8 25 0f (11110000 10101000 00100101 00001111) (254126320)
4 4 (object header) 00 70 00 00 (00000000 01110000 00000000 00000000) (28672)
8 4 (object header) b4 02 01 f8 (10110100 00000010 00000001 11111000) (-134151500)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
<<============= Thread[Thread-2,5,main] ROUND-0 ==============>>
org.reion.Student 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) b4 02 01 f8 (10110100 00000010 00000001 11111000) (-134151500)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
<<============= Thread[Thread-3,5,main] ROUND-0 ==============>>
org.reion.Student object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) f0 d8 35 0f (11110000 11011000 00110101 00001111) (255187184)
4 4 (object header) 00 70 00 00 (00000000 01110000 00000000 00000000) (28672)
8 4 (object header) b4 02 01 f8 (10110100 00000010 00000001 11111000) (-134151500)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
<<============= Thread[Thread-3,5,main] ROUND-1 ==============>>
org.reion.Student object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 5a d1 82 44 (01011010 11010001 10000010 01000100) (1149423962)
4 4 (object header) dd 7f 00 00 (11011101 01111111 00000000 00000000) (32733)
8 4 (object header) b4 02 01 f8 (10110100 00000010 00000001 11111000) (-134151500)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
<<============= Thread[Thread-2,5,main] ROUND-1 ==============>>
org.reion.Student object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 5a d1 82 44 (01011010 11010001 10000010 01000100) (1149423962)
4 4 (object header) dd 7f 00 00 (11011101 01111111 00000000 00000000) (32733)
8 4 (object header) b4 02 01 f8 (10110100 00000010 00000001 11111000) (-134151500)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
【【【【【【【【中间打印省略】】】】】】】】】
退出线程：Thread[Thread-3,5,main]
<<============= Thread[Thread-2,5,main] ROUND-8 ==============>>
org.reion.Student object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) f0 a8 25 0f (11110000 10101000 00100101 00001111) (254126320)
4 4 (object header) 00 70 00 00 (00000000 01110000 00000000 00000000) (28672)
8 4 (object header) b4 02 01 f8 (10110100 00000010 00000001 11111000) (-134151500)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
<<============= Thread[Thread-2,5,main] ROUND-8 ==============>>
org.reion.Student 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) b4 02 01 f8 (10110100 00000010 00000001 11111000) (-134151500)
12 4 java.lang.String Student.firstName null
16 4 java.lang.String Student.lastName null
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
退出线程：Thread[Thread-2,5,main]
参考资料
Synchronization Created by Christian Wimmer, last modified on Apr 29, 2008
Compressed Ordinary Object Pointer
markOop.hpp
What is in java object header
脚注
压缩的类指针 (Compressed Class Pointer)，使用虚拟机参数
-XX:-UseCompressedClassPointers关闭压缩，默认开启。压缩的普通对象指针 (Compressed Ordinary Object Pointer)，使用虚拟机参数
-XX:-UseCompressedOops关闭压缩，默认开启。How does the default hashCode() work?
source：https://reionchan.github.io/2020/08/26/about-synchronized-lock-optimization/
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