Syncronized原理
一个简单的例子来演示 Syncronized 的作用
class BankAccount {
private int balance = 0; // 账户余额
// 存钱
public synchronized void deposit(int amount) {
balance += amount;
System.out.println(Thread.currentThread().getName() + " 存入: " + amount + "，当前余额: " + balance);
}
// 取钱
public synchronized void withdraw(int amount) {
if (balance >= amount) {
balance -= amount;
System.out.println(Thread.currentThread().getName() + " 取出: " + amount + "，当前余额: " + balance);
} else {
System.out.println(Thread.currentThread().getName() + " 取钱失败，余额不足，当前余额: " + balance);
}
}
}
public class SynchronizedExample {
public static void main(String[] args) {
BankAccount account = new BankAccount();
// 模拟多线程操作
Thread t1 = new Thread(() -> {
account.deposit(100);
account.withdraw(50);
}, "线程1");
t1.start();
Thread t2 = new Thread(() -> {
account.deposit(200);
account.withdraw(150);
}, "线程2");
t2.start();
Thread t3 = new Thread(() -> {
account.withdraw(50);
}, "线程3");
t3.start();
}
}
从肉眼看，线程1操作完剩50，线程2操作完剩100，线程3操作完剩50。执行代码后，
输出结果：
线程1 存入: 100，当前余额: 100
线程1 取出: 50，当前余额: 50
线程2 存入: 200，当前余额: 250
线程2 取出: 150，当前余额: 100
线程3 取出: 50，当前余额: 50
输出结果符合预期，其中deposit和withdraw方法都是同步方法，使用synchronized关键字修饰，这样就保证了在多线程环墋下，同一时刻只有一个线程可以访问这两个方法。
如果去掉 synchronized 关键字，那么输出结果可能会是：
线程1 存入: 100，当前余额: 100
线程1 取出: 50，当前余额: 250
线程2 存入: 200，当前余额: 300
线程2 取出: 150，当前余额: 50
线程3 取出: 50，当前余额: 200
执行了很多次，即使没有 synchronized，结果可能看起来是“正确的”，但这是偶然的，不能保证在所有情况下都正确。
原因1: 线程切换的不可控性
多线程的执行顺序和切换点是不确定的，可能正好每次线程切换都“幸运地”避免了冲突。
原因2：线程冲突的表现依赖于具体环境
在简单的场景（如两三个线程）中，冲突的概率低，但随着线程数和操作的复杂度增加，冲突导致的不正确结果的概率会显著提高。
查看源码
使用 javac SynchronizedExample.java 可以看到编译后的字节码文件 SynchronizedExample.class 和 BackAccount.class，内容和源代码没有太大区别。
接下来使用 javap -c BackAccount.class 查看编译后的字节码文件，可以看到 synchronized 关键字在字节码中的表现：
BankAccount.class
Compiled from "SynchronizedExample.java"
class top.gclhaha.kafka.sync.BankAccount {
top.gclhaha.kafka.sync.BankAccount();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: aload_0
5: iconst_0
6: putfield #2 // Field balance:I
9: return
public synchronized void deposit(int);
Code:
0: aload_0
1: dup
2: getfield #2 // Field balance:I
5: iload_1
6: iadd
7: putfield #2 // Field balance:I
10: getstatic #3 // Field java/lang/System.out:Ljava/io/PrintStream;
13: new #4 // class java/lang/StringBuilder
16: dup
17: invokespecial #5 // Method java/lang/StringBuilder."<init>":()V
20: invokestatic #6 // Method java/lang/Thread.currentThread:()Ljava/lang/Thread;
23: invokevirtual #7 // Method java/lang/Thread.getName:()Ljava/lang/String;
26: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
29: ldc #9 // String 存入:
31: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
34: iload_1
35: invokevirtual #10 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
38: ldc #11 // String ，当前余额:
40: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
43: aload_0
44: getfield #2 // Field balance:I
47: invokevirtual #10 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
50: invokevirtual #12 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
53: invokevirtual #13 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
56: return
public synchronized void withdraw(int);
Code:
0: aload_0
1: getfield #2 // Field balance:I
4: iload_1
5: if_icmplt 67
8: aload_0
9: dup
10: getfield #2 // Field balance:I
13: iload_1
14: isub
15: putfield #2 // Field balance:I
18: getstatic #3 // Field java/lang/System.out:Ljava/io/PrintStream;
21: new #4 // class java/lang/StringBuilder
24: dup
25: invokespecial #5 // Method java/lang/StringBuilder."<init>":()V
28: invokestatic #6 // Method java/lang/Thread.currentThread:()Ljava/lang/Thread;
31: invokevirtual #7 // Method java/lang/Thread.getName:()Ljava/lang/String;
34: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
37: ldc #14 // String 取出:
39: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
42: iload_1
43: invokevirtual #10 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
46: ldc #11 // String ，当前余额:
48: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
51: aload_0
52: getfield #2 // Field balance:I
55: invokevirtual #10 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
58: invokevirtual #12 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
61: invokevirtual #13 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
64: goto 104
67: getstatic #3 // Field java/lang/System.out:Ljava/io/PrintStream;
70: new #4 // class java/lang/StringBuilder
73: dup
74: invokespecial #5 // Method java/lang/StringBuilder."<init>":()V
77: invokestatic #6 // Method java/lang/Thread.currentThread:()Ljava/lang/Thread;
80: invokevirtual #7 // Method java/lang/Thread.getName:()Ljava/lang/String;
83: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
86: ldc #15 // String 取钱失败，余额不足，当前余额:
88: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
91: aload_0
92: getfield #2 // Field balance:I
95: invokevirtual #10 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
98: invokevirtual #12 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
101: invokevirtual #13 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
104: return
}
可以方法上有 synchronized 关键字
synchronized 方法在字节码中没有显式的 monitorenter 和 monitorexit 指令。
JVM 会通过 ACC_SYNCHRONIZED 标志来保证方法的同步。
可以通过反汇编工具查看该标志，
接下来使用 javap -v BackAccount.class 查看
BankAccount.class
Classfile /Users/gclhaha/Documents/develop/javaworkspace/github/kafka_demo/src/main/java/top/gclhaha/kafka/sync/BankAccount.class
Last modified 2024-12-5; size 1098 bytes
MD5 checksum dfee83f2ab249c9d791a00ae0b9a0b47
Compiled from "SynchronizedExample.java"
class top.gclhaha.kafka.sync.BankAccount
minor version: 0
major version: 52
flags: ACC_SUPER
Constant pool:
#1 = Methodref #17.#30 // java/lang/Object."<init>":()V
#2 = Fieldref #16.#31 // top/gclhaha/kafka/sync/BankAccount.balance:I
#3 = Fieldref #32.#33 // java/lang/System.out:Ljava/io/PrintStream;
#4 = Class #34 // java/lang/StringBuilder
#5 = Methodref #4.#30 // java/lang/StringBuilder."<init>":()V
#6 = Methodref #35.#36 // java/lang/Thread.currentThread:()Ljava/lang/Thread;
#7 = Methodref #35.#37 // java/lang/Thread.getName:()Ljava/lang/String;
#8 = Methodref #4.#38 // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#9 = String #39 // 存入:
#10 = Methodref #4.#40 // java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
#11 = String #41 // ，当前余额:
#12 = Methodref #4.#42 // java/lang/StringBuilder.toString:()Ljava/lang/String;
#13 = Methodref #43.#44 // java/io/PrintStream.println:(Ljava/lang/String;)V
#14 = String #45 // 取出:
#15 = String #46 // 取钱失败，余额不足，当前余额:
#16 = Class #47 // top/gclhaha/kafka/sync/BankAccount
#17 = Class #48 // java/lang/Object
#18 = Utf8 balance
#19 = Utf8 I
#20 = Utf8 <init>
#21 = Utf8 ()V
#22 = Utf8 Code
#23 = Utf8 LineNumberTable
#24 = Utf8 deposit
#25 = Utf8 (I)V
#26 = Utf8 withdraw
#27 = Utf8 StackMapTable
#28 = Utf8 SourceFile
#29 = Utf8 SynchronizedExample.java
#30 = NameAndType #20:#21 // "<init>":()V
#31 = NameAndType #18:#19 // balance:I
#32 = Class #49 // java/lang/System
#33 = NameAndType #50:#51 // out:Ljava/io/PrintStream;
#34 = Utf8 java/lang/StringBuilder
#35 = Class #52 // java/lang/Thread
#36 = NameAndType #53:#54 // currentThread:()Ljava/lang/Thread;
#37 = NameAndType #55:#56 // getName:()Ljava/lang/String;
#38 = NameAndType #57:#58 // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#39 = Utf8 存入:
#40 = NameAndType #57:#59 // append:(I)Ljava/lang/StringBuilder;
#41 = Utf8 ，当前余额:
#42 = NameAndType #60:#56 // toString:()Ljava/lang/String;
#43 = Class #61 // java/io/PrintStream
#44 = NameAndType #62:#63 // println:(Ljava/lang/String;)V
#45 = Utf8 取出:
#46 = Utf8 取钱失败，余额不足，当前余额:
#47 = Utf8 top/gclhaha/kafka/sync/BankAccount
#48 = Utf8 java/lang/Object
#49 = Utf8 java/lang/System
#50 = Utf8 out
#51 = Utf8 Ljava/io/PrintStream;
#52 = Utf8 java/lang/Thread
#53 = Utf8 currentThread
#54 = Utf8 ()Ljava/lang/Thread;
#55 = Utf8 getName
#56 = Utf8 ()Ljava/lang/String;
#57 = Utf8 append
#58 = Utf8 (Ljava/lang/String;)Ljava/lang/StringBuilder;
#59 = Utf8 (I)Ljava/lang/StringBuilder;
#60 = Utf8 toString
#61 = Utf8 java/io/PrintStream
#62 = Utf8 println
#63 = Utf8 (Ljava/lang/String;)V
{
top.gclhaha.kafka.sync.BankAccount();
descriptor: ()V
flags:
Code:
stack=2, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: aload_0
5: iconst_0
6: putfield #2 // Field balance:I
9: return
LineNumberTable:
line 4: 0
line 5: 4
public synchronized void deposit(int);
descriptor: (I)V
flags: ACC_PUBLIC, ACC_SYNCHRONIZED
Code:
stack=3, locals=2, args_size=2
0: aload_0
1: dup
2: getfield #2 // Field balance:I
5: iload_1
6: iadd
7: putfield #2 // Field balance:I
10: getstatic #3 // Field java/lang/System.out:Ljava/io/PrintStream;
13: new #4 // class java/lang/StringBuilder
16: dup
17: invokespecial #5 // Method java/lang/StringBuilder."<init>":()V
20: invokestatic #6 // Method java/lang/Thread.currentThread:()Ljava/lang/Thread;
23: invokevirtual #7 // Method java/lang/Thread.getName:()Ljava/lang/String;
26: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
29: ldc #9 // String 存入:
31: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
34: iload_1
35: invokevirtual #10 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
38: ldc #11 // String ，当前余额:
40: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
43: aload_0
44: getfield #2 // Field balance:I
47: invokevirtual #10 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
50: invokevirtual #12 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
53: invokevirtual #13 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
56: return
LineNumberTable:
line 9: 0
line 10: 10
line 11: 56
public synchronized void withdraw(int);
descriptor: (I)V
flags: ACC_PUBLIC, ACC_SYNCHRONIZED
Code:
stack=3, locals=2, args_size=2
0: aload_0
1: getfield #2 // Field balance:I
4: iload_1
5: if_icmplt 67
8: aload_0
9: dup
10: getfield #2 // Field balance:I
13: iload_1
14: isub
15: putfield #2 // Field balance:I
18: getstatic #3 // Field java/lang/System.out:Ljava/io/PrintStream;
21: new #4 // class java/lang/StringBuilder
24: dup
25: invokespecial #5 // Method java/lang/StringBuilder."<init>":()V
28: invokestatic #6 // Method java/lang/Thread.currentThread:()Ljava/lang/Thread;
31: invokevirtual #7 // Method java/lang/Thread.getName:()Ljava/lang/String;
34: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
37: ldc #14 // String 取出:
39: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
42: iload_1
43: invokevirtual #10 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
46: ldc #11 // String ，当前余额:
48: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
51: aload_0
52: getfield #2 // Field balance:I
55: invokevirtual #10 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
58: invokevirtual #12 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
61: invokevirtual #13 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
64: goto 104
67: getstatic #3 // Field java/lang/System.out:Ljava/io/PrintStream;
70: new #4 // class java/lang/StringBuilder
73: dup
74: invokespecial #5 // Method java/lang/StringBuilder."<init>":()V
77: invokestatic #6 // Method java/lang/Thread.currentThread:()Ljava/lang/Thread;
80: invokevirtual #7 // Method java/lang/Thread.getName:()Ljava/lang/String;
83: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
86: ldc #15 // String 取钱失败，余额不足，当前余额:
88: invokevirtual #8 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
91: aload_0
92: getfield #2 // Field balance:I
95: invokevirtual #10 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
98: invokevirtual #12 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
101: invokevirtual #13 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
104: return
LineNumberTable:
line 15: 0
line 16: 8
line 17: 18
line 19: 67
line 21: 104
StackMapTable: number_of_entries = 2
frame_type = 251 /* same_frame_extended */
offset_delta = 67
frame_type = 36 /* same */
}
SourceFile: "SynchronizedExample.java"
可以看到方法上有 ACC_SYNCHRONIZED 标志。
对于带有 ACC_SYNCHRONIZED 标志的方法，JVM 在方法调用和返回时，会自动进行以下操作：
- 进入方法时：加锁 JVM 自动调用 monitorenter，尝试获取当前对象的 Monitor 锁。
- 退出方法时：解锁 JVM 自动调用 monitorexit，释放锁资源。
这部分逻辑由 JVM 的执行引擎实现，无需在字节码中显式插入 monitorenter 和 monitorexit 指令。
JVM 内部的监视器锁（Monitor）机制
Java 中的 synchronized 是通过 JVM 的监视器锁（Monitor）实现的。每个对象都关联着一个 Monitor，它在对象头（Object Header）中记录了锁的状态。以下是几个核心概念：
1.1 对象头结构
Java 对象的内存布局中包含一个 对象头（Object Header）：
• Mark Word：包含锁标志、偏向线程 ID、锁计数等信息。
• Class Pointer：指向对象所属的类的元信息。
当对象被用作锁时，Mark Word 的内容会变化以反映当前锁的状态。
1.2 Monitor 的基本操作
监视器锁的核心操作包括：
monitorenter：
• 线程试图获取对象的 Monitor。
• 如果 Monitor 当前未被占用，线程成功获取锁。
• 如果 Monitor 被其他线程占用，当前线程进入阻塞状态，等待锁的释放。
monitorexit：
• 释放当前持有的 Monitor。
• 如果有线程在等待此 Monitor，通知其中一个线程获取锁。
JVM 中每个 Monitor 是由 C++ 实现的 ObjectMonitor 对象管理。
1.3 JVM 锁的实现
偏向锁（Biased Locking）
• 原理：如果一个线程多次访问某个锁，对其进行偏向优化，减少 CAS（Compare-And-Swap）操作。
• 实现：当线程第一次获取锁时，将其线程 ID 写入对象头的 Mark Word。后续该线程获取锁时，不需要进行同步操作，直接进入临界区。
• 退出偏向锁：如果另一个线程尝试获取锁，则撤销偏向锁，并升级为轻量级锁。
轻量级锁（Lightweight Locking）
• 原理：通过 CAS 操作，减少重量级锁的线程阻塞。
• 实现：线程获取锁时，创建一个锁记录（Lock Record）并将其存储在当前线程的栈帧中。使用 CAS 将对象头中的 Mark Word 替换为指向锁记录的指针。
• 锁竞争：如果多个线程尝试获取锁，则升级为重量级锁。
重量级锁（Heavyweight Locking）
• 原理：依赖操作系统的互斥机制（Mutex），对线程进行阻塞和唤醒。
• 实现：当轻量级锁竞争失败时，进入重量级锁状态。操作系统通过线程切换调度保证互斥，代价较高。
javap 常见选项
• -c：反编译成字节码指令。
• -v：显示详细信息，包括类的标志、方法标志等。
• -l：显示局部变量表。
• -s：显示方法的签名信息。
和 Lock 的区别
使用Lock来实现一遍，代码如下：
import java.util.concurrent.locks.ReentrantLock;
class BankAccount {
private final ReentrantLock lock = new ReentrantLock();
private int balance = 0; // 账户余额
// 存钱
public void deposit(int amount) {
lock.lock();
try {
balance += amount;
System.out.println(Thread.currentThread().getName() + " 存入: " + amount + "，当前余额: " + balance);
} finally {
lock.unlock();
}
}
// 取钱
public void withdraw(int amount) {
lock.lock();
try {
if (balance >= amount) {
balance -= amount;
System.out.println(Thread.currentThread().getName() + " 取出: " + amount + "，当前余额: " + balance);
} else {
System.out.println(Thread.currentThread().getName() + " 取钱失败，余额不足，当前余额: " + balance);
}
} finally {
lock.unlock();
}
}
}
public class LockSyncExample {
public static void main(String[] args) {
BankAccount account = new BankAccount();
// 模拟多线程操作
Thread t1 = new Thread(() -> {
account.deposit(100);
account.withdraw(50);
}, "线程1");
t1.start();
Thread t2 = new Thread(() -> {
account.deposit(200);
account.withdraw(150);
}, "线程2");
t2.start();
Thread t3 = new Thread(() -> {
account.withdraw(50);
}, "线程3");
t3.start();
}
}
使用 javac LockSyncExample.java 编译后，使用 javap -c BankAccount.class 查看编译后的字节码文件，可以看到 ReentrantLock 的使用：
Classfile /Users/gclhaha/Documents/develop/javaworkspace/github/kafka_demo/src/main/java/top/gclhaha/kafka/locksync/BankAccount.class
Last modified 2024-12-7; size 1418 bytes
MD5 checksum 26f83c8ff22cf8e47110e9fc48cc6a3d
Compiled from "LockSyncExample.java"
class top.gclhaha.kafka.locksync.BankAccount
minor version: 0
major version: 52
flags: ACC_SUPER
Constant pool:
#1 = Methodref #22.#38 // java/lang/Object."<init>":()V
#2 = Class #39 // java/util/concurrent/locks/ReentrantLock
#3 = Methodref #2.#38 // java/util/concurrent/locks/ReentrantLock."<init>":()V
#4 = Fieldref #21.#40 // top/gclhaha/kafka/locksync/BankAccount.lock:Ljava/util/concurrent/locks/ReentrantLock;
#5 = Fieldref #21.#41 // top/gclhaha/kafka/locksync/BankAccount.balance:I
#6 = Methodref #2.#42 // java/util/concurrent/locks/ReentrantLock.lock:()V
#7 = Fieldref #43.#44 // java/lang/System.out:Ljava/io/PrintStream;
#8 = Class #45 // java/lang/StringBuilder
#9 = Methodref #8.#38 // java/lang/StringBuilder."<init>":()V
#10 = Methodref #46.#47 // java/lang/Thread.currentThread:()Ljava/lang/Thread;
#11 = Methodref #46.#48 // java/lang/Thread.getName:()Ljava/lang/String;
#12 = Methodref #8.#49 // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#13 = String #50 // 存入:
#14 = Methodref #8.#51 // java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
#15 = String #52 // ，当前余额:
#16 = Methodref #8.#53 // java/lang/StringBuilder.toString:()Ljava/lang/String;
#17 = Methodref #54.#55 // java/io/PrintStream.println:(Ljava/lang/String;)V
#18 = Methodref #2.#56 // java/util/concurrent/locks/ReentrantLock.unlock:()V
#19 = String #57 // 取出:
#20 = String #58 // 取钱失败，余额不足，当前余额:
#21 = Class #59 // top/gclhaha/kafka/locksync/BankAccount
#22 = Class #60 // java/lang/Object
#23 = Utf8 lock
#24 = Utf8 Ljava/util/concurrent/locks/ReentrantLock;
#25 = Utf8 balance
#26 = Utf8 I
#27 = Utf8 <init>
#28 = Utf8 ()V
#29 = Utf8 Code
#30 = Utf8 LineNumberTable
#31 = Utf8 deposit
#32 = Utf8 (I)V
#33 = Utf8 StackMapTable
#34 = Class #61 // java/lang/Throwable
#35 = Utf8 withdraw
#36 = Utf8 SourceFile
#37 = Utf8 LockSyncExample.java
#38 = NameAndType #27:#28 // "<init>":()V
#39 = Utf8 java/util/concurrent/locks/ReentrantLock
#40 = NameAndType #23:#24 // lock:Ljava/util/concurrent/locks/ReentrantLock;
#41 = NameAndType #25:#26 // balance:I
#42 = NameAndType #23:#28 // lock:()V
#43 = Class #62 // java/lang/System
#44 = NameAndType #63:#64 // out:Ljava/io/PrintStream;
#45 = Utf8 java/lang/StringBuilder
#46 = Class #65 // java/lang/Thread
#47 = NameAndType #66:#67 // currentThread:()Ljava/lang/Thread;
#48 = NameAndType #68:#69 // getName:()Ljava/lang/String;
#49 = NameAndType #70:#71 // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#50 = Utf8 存入:
#51 = NameAndType #70:#72 // append:(I)Ljava/lang/StringBuilder;
#52 = Utf8 ，当前余额:
#53 = NameAndType #73:#69 // toString:()Ljava/lang/String;
#54 = Class #74 // java/io/PrintStream
#55 = NameAndType #75:#76 // println:(Ljava/lang/String;)V
#56 = NameAndType #77:#28 // unlock:()V
#57 = Utf8 取出:
#58 = Utf8 取钱失败，余额不足，当前余额:
#59 = Utf8 top/gclhaha/kafka/locksync/BankAccount
#60 = Utf8 java/lang/Object
#61 = Utf8 java/lang/Throwable
#62 = Utf8 java/lang/System
#63 = Utf8 out
#64 = Utf8 Ljava/io/PrintStream;
#65 = Utf8 java/lang/Thread
#66 = Utf8 currentThread
#67 = Utf8 ()Ljava/lang/Thread;
#68 = Utf8 getName
#69 = Utf8 ()Ljava/lang/String;
#70 = Utf8 append
#71 = Utf8 (Ljava/lang/String;)Ljava/lang/StringBuilder;
#72 = Utf8 (I)Ljava/lang/StringBuilder;
#73 = Utf8 toString
#74 = Utf8 java/io/PrintStream
#75 = Utf8 println
#76 = Utf8 (Ljava/lang/String;)V
#77 = Utf8 unlock
{
top.gclhaha.kafka.locksync.BankAccount();
descriptor: ()V
flags:
Code:
stack=3, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: aload_0
5: new #2 // class java/util/concurrent/locks/ReentrantLock
8: dup
9: invokespecial #3 // Method java/util/concurrent/locks/ReentrantLock."<init>":()V
12: putfield #4 // Field lock:Ljava/util/concurrent/locks/ReentrantLock;
15: aload_0
16: iconst_0
17: putfield #5 // Field balance:I
20: return
LineNumberTable:
line 6: 0
line 8: 4
line 10: 15
public void deposit(int);
descriptor: (I)V
flags: ACC_PUBLIC
Code:
stack=3, locals=3, args_size=2
0: aload_0
1: getfield #4 // Field lock:Ljava/util/concurrent/locks/ReentrantLock;
4: invokevirtual #6 // Method java/util/concurrent/locks/ReentrantLock.lock:()V
7: aload_0
8: dup
9: getfield #5 // Field balance:I
12: iload_1
13: iadd
14: putfield #5 // Field balance:I
17: getstatic #7 // Field java/lang/System.out:Ljava/io/PrintStream;
20: new #8 // class java/lang/StringBuilder
23: dup
24: invokespecial #9 // Method java/lang/StringBuilder."<init>":()V
27: invokestatic #10 // Method java/lang/Thread.currentThread:()Ljava/lang/Thread;
30: invokevirtual #11 // Method java/lang/Thread.getName:()Ljava/lang/String;
33: invokevirtual #12 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
36: ldc #13 // String 存入:
38: invokevirtual #12 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
41: iload_1
42: invokevirtual #14 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
45: ldc #15 // String ，当前余额:
47: invokevirtual #12 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
50: aload_0
51: getfield #5 // Field balance:I
54: invokevirtual #14 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
57: invokevirtual #16 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
60: invokevirtual #17 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
63: aload_0
64: getfield #4 // Field lock:Ljava/util/concurrent/locks/ReentrantLock;
67: invokevirtual #18 // Method java/util/concurrent/locks/ReentrantLock.unlock:()V
70: goto 83
73: astore_2
74: aload_0
75: getfield #4 // Field lock:Ljava/util/concurrent/locks/ReentrantLock;
78: invokevirtual #18 // Method java/util/concurrent/locks/ReentrantLock.unlock:()V
81: aload_2
82: athrow
83: return
Exception table:
from to target type
7 63 73 any
LineNumberTable:
line 14: 0
line 16: 7
line 17: 17
line 19: 63
line 20: 70
line 19: 73
line 20: 81
line 22: 83
StackMapTable: number_of_entries = 2
frame_type = 247 /* same_locals_1_stack_item_frame_extended */
offset_delta = 73
stack = [ class java/lang/Throwable ]
frame_type = 9 /* same */
public void withdraw(int);
descriptor: (I)V
flags: ACC_PUBLIC
Code:
stack=3, locals=3, args_size=2
0: aload_0
1: getfield #4 // Field lock:Ljava/util/concurrent/locks/ReentrantLock;
4: invokevirtual #6 // Method java/util/concurrent/locks/ReentrantLock.lock:()V
7: aload_0
8: getfield #5 // Field balance:I
11: iload_1
12: if_icmplt 74
15: aload_0
16: dup
17: getfield #5 // Field balance:I
20: iload_1
21: isub
22: putfield #5 // Field balance:I
25: getstatic #7 // Field java/lang/System.out:Ljava/io/PrintStream;
28: new #8 // class java/lang/StringBuilder
31: dup
32: invokespecial #9 // Method java/lang/StringBuilder."<init>":()V
35: invokestatic #10 // Method java/lang/Thread.currentThread:()Ljava/lang/Thread;
38: invokevirtual #11 // Method java/lang/Thread.getName:()Ljava/lang/String;
41: invokevirtual #12 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
44: ldc #19 // String 取出:
46: invokevirtual #12 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
49: iload_1
50: invokevirtual #14 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
53: ldc #15 // String ，当前余额:
55: invokevirtual #12 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
58: aload_0
59: getfield #5 // Field balance:I
62: invokevirtual #14 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
65: invokevirtual #16 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
68: invokevirtual #17 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
71: goto 111
74: getstatic #7 // Field java/lang/System.out:Ljava/io/PrintStream;
77: new #8 // class java/lang/StringBuilder
80: dup
81: invokespecial #9 // Method java/lang/StringBuilder."<init>":()V
84: invokestatic #10 // Method java/lang/Thread.currentThread:()Ljava/lang/Thread;
87: invokevirtual #11 // Method java/lang/Thread.getName:()Ljava/lang/String;
90: invokevirtual #12 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
93: ldc #20 // String 取钱失败，余额不足，当前余额:
95: invokevirtual #12 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
98: aload_0
99: getfield #5 // Field balance:I
102: invokevirtual #14 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
105: invokevirtual #16 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
108: invokevirtual #17 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
111: aload_0
112: getfield #4 // Field lock:Ljava/util/concurrent/locks/ReentrantLock;
115: invokevirtual #18 // Method java/util/concurrent/locks/ReentrantLock.unlock:()V
118: goto 131
121: astore_2
122: aload_0
123: getfield #4 // Field lock:Ljava/util/concurrent/locks/ReentrantLock;
126: invokevirtual #18 // Method java/util/concurrent/locks/ReentrantLock.unlock:()V
129: aload_2
130: athrow
131: return
Exception table:
from to target type
7 111 121 any
LineNumberTable:
line 26: 0
line 28: 7
line 29: 15
line 30: 25
line 32: 74
line 35: 111
line 36: 118
line 35: 121
line 36: 129
line 37: 131
StackMapTable: number_of_entries = 4
frame_type = 251 /* same_frame_extended */
offset_delta = 74
frame_type = 36 /* same */
frame_type = 73 /* same_locals_1_stack_item */
stack = [ class java/lang/Throwable ]
frame_type = 9 /* same */
}
SourceFile: "LockSyncExample.java"
其中可以看到 ReentrantLock 的使用。invokevirtual 指令调用 ReentrantLock 的 lock 和 unlock 方法。
字节码区别
syncronized：使用了 monitorenter 和 monitorexit 指令实现同步。JVM 本身管理锁的进入与退出，保证锁释放的安全性。
ReentrantLock：调用了 lock.lock() 和 lock.unlock() 的方法。需要显式地调用 unlock()，否则可能因异常未释放锁，导致死锁风险。
方法标记区别
synchronized：方法会带有 ACC_SYNCHRONIZED 标志，JVM 在调用时会自动获取和释放锁。
ReentrantLock：没有任何方法级别的字节码标志，锁操作是通过字节码调用 ReentrantLock 类的方法实现。
异常处理
synchronized：JVM 自动生成异常表，确保在任何异常情况下，monitorexit 都能被执行。
ReentrantLock：异常处理由程序员控制。开需要使用 try-finally 保证 unlock() 一定被调用。
总结
Lock 提供更细粒度的锁控制和更高的性能（比如可中断锁、定时尝试锁等），但使用 synchronized 时 JVM 会进行一些优化，比如偏向锁、轻量级锁等。
Lock 比 synchronized 更灵活，但需要手动释放锁，代码复杂性略高。
使用 synchronized 时 JVM 自动管理锁的获取和释放，更容易避免编程错误。