synchronized关键字的作用是什么？
Java 中关键字 synchronized 表示只有一个线程可以获取作用对象的锁，执行代码，阻塞其他线程。
作用：
- 确保线程互斥地访问同步代码
- 保证共享变量的修改能够及时可见
- 有效解决重排序问题
用法：
- 修饰普通方法
- 修饰静态方法
- 指定对象，修饰代码块
特点：
- 阻塞未获取到锁、竞争同一个对象锁的线程
- 获取锁无法设置超时
- 无法实现公平锁
- 控制等待和唤醒需要结合加锁对象的 wait() 和 notify()、notifyAll()
- 锁的功能是 JVM 层面实现的
- 在加锁代码块执行完或者出现异常，自动释放锁
原理：
- 同步代码块是通过 monitorenter 和 monitorexit 指令获取线程的执行权
- 同步方法通过加 ACC_SYNCHRONIZED 标识实现线程的执行权的控制
测试代码：
public class TestSynchronized {
public void sync() {
synchronized (this) {
System.out.println("sync");
}
}
public synchronized void syncdo() {
System.out.println("syncdo");
}
public static synchronized void staticSyncdo() {
System.out.println("staticSyncdo");
}
}
通过JDK 反汇编指令 javap -c -v TestSynchronized
javap -c -v TestSynchronized
Last modified 2019-5-27; size 719 bytes
MD5 checksum e5058a43e76fe1cff6748d4eb1565658
Compiled from "TestSynchronized.java"
public class constxiong.interview.TestSynchronized
minor version: 0
major version: 49
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Class #2 // constxiong/interview/TestSynchronized
#2 = Utf8 constxiong/interview/TestSynchronized
#3 = Class #4 // java/lang/Object
#4 = Utf8 java/lang/Object
#5 = Utf8 <init>
#6 = Utf8 ()V
#7 = Utf8 Code
#8 = Methodref #3.#9 // java/lang/Object."<init>":()V
#9 = NameAndType #5:#6 // "<init>":()V
#10 = Utf8 LineNumberTable
#11 = Utf8 LocalVariableTable
#12 = Utf8 this
#13 = Utf8 Lconstxiong/interview/TestSynchronized;
#14 = Utf8 sync
#15 = Fieldref #16.#18 // java/lang/System.out:Ljava/io/PrintStream;
#16 = Class #17 // java/lang/System
#17 = Utf8 java/lang/System
#18 = NameAndType #19:#20 // out:Ljava/io/PrintStream;
#19 = Utf8 out
#20 = Utf8 Ljava/io/PrintStream;
#21 = String #14 // sync
#22 = Methodref #23.#25 // java/io/PrintStream.println:(Ljava/lang/String;)V
#23 = Class #24 // java/io/PrintStream
#24 = Utf8 java/io/PrintStream
#25 = NameAndType #26:#27 // println:(Ljava/lang/String;)V
#26 = Utf8 println
#27 = Utf8 (Ljava/lang/String;)V
#28 = Utf8 syncdo
#29 = String #28 // syncdo
#30 = Utf8 staticSyncdo
#31 = String #30 // staticSyncdo
#32 = Utf8 SourceFile
#33 = Utf8 TestSynchronized.java
{
public constxiong.interview.TestSynchronized();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #8 // Method java/lang/Object."<init>":()V
4: return
LineNumberTable:
line 3: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lconstxiong/interview/TestSynchronized;
public void sync();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=2, locals=2, args_size=1
0: aload_0
1: dup
2: astore_1
3: monitorenter
4: getstatic #15 // Field java/lang/System.out:Ljava/io/PrintStream;
7: ldc #21 // String sync
9: invokevirtual #22 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
12: aload_1
13: monitorexit
14: goto 20
17: aload_1
18: monitorexit
19: athrow
20: return
Exception table:
from to target type
4 14 17 any
17 19 17 any
LineNumberTable:
line 6: 0
line 7: 4
line 6: 12
line 9: 20
LocalVariableTable:
Start Length Slot Name Signature
0 21 0 this Lconstxiong/interview/TestSynchronized;
public synchronized void syncdo();
descriptor: ()V
flags: ACC_PUBLIC, ACC_SYNCHRONIZED
Code:
stack=2, locals=1, args_size=1
0: getstatic #15 // Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #29 // String syncdo
5: invokevirtual #22 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
LineNumberTable:
line 12: 0
line 13: 8
LocalVariableTable:
Start Length Slot Name Signature
0 9 0 this Lconstxiong/interview/TestSynchronized;
public static synchronized void staticSyncdo();
descriptor: ()V
flags: ACC_PUBLIC, ACC_STATIC, ACC_SYNCHRONIZED
Code:
stack=2, locals=0, args_size=0
0: getstatic #15 // Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #31 // String staticSyncdo
5: invokevirtual #22 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
LineNumberTable:
line 16: 0
line 17: 8
LocalVariableTable:
Start Length Slot Name Signature
}
SourceFile: "TestSynchronized.java"
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