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首先synchronized是阻塞式锁，悲观锁，会造成线程的挂起和恢复操作。JVM JDK1.2以后的线程模型是用户态和内核态共同存在，存在切换开销。

先看段代码

public class Test2 {    public synchronized void test() {        System.out.println("hello");    }    public void test3() {        synchronized (this) {            System.out.println("hello");        }    }    public static synchronized void test2() {        System.out.println("hello");    }}

常用用法，修饰对象，方法，静态方法

看原理

使用javap -c xx.class > 1.txt

public synchronized void test();    Code:       0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;       3: ldc           #3                  // String hello       5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V       8: return  public void test3();    Code:       0: aload_0       1: dup       2: astore_1       3: monitorenter       4: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;       7: ldc           #3                  // String hello       9: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V      12: aload_1      13: monitorexit      14: goto          22      17: astore_2      18: aload_1      19: monitorexit      20: aload_2      21: athrow      22: return    Exception table:       from    to  target type           4    14    17   any          17    20    17   any  public static synchronized void test2();    Code:       0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;       3: ldc           #3                  // String hello       5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V       8: return

通过javap -v即可显示详情

Classfile /Users/huahua/IdeaProjects/web-filter/target/classes/com/feng/demo/Test.class  Last modified 2020-5-6; size 731 bytes  MD5 checksum caa59f5b81fc01bdb2e60cd740a60c5a  Compiled from "Test.java"public class com.feng.demo.Test  minor version: 0  major version: 52  flags: ACC_PUBLIC, ACC_SUPERConstant pool:   #1 = Methodref          #6.#23         // java/lang/Object."
   
    ":()V   #2 = Fieldref           #24.#25        // java/lang/System.out:Ljava/io/PrintStream;   #3 = String             #26            // hello   #4 = Methodref          #27.#28        // java/io/PrintStream.println:(Ljava/lang/String;)V   #5 = Class              #29            // com/feng/demo/Test   #6 = Class              #30            // java/lang/Object   #7 = Utf8               
    
        #8 = Utf8               ()V   #9 = Utf8               Code  #10 = Utf8               LineNumberTable  #11 = Utf8               LocalVariableTable  #12 = Utf8               this  #13 = Utf8               Lcom/feng/demo/Test;  #14 = Utf8               test  #15 = Utf8               test3  #16 = Utf8               StackMapTable  #17 = Class              #29            // com/feng/demo/Test  #18 = Class              #30            // java/lang/Object  #19 = Class              #31            // java/lang/Throwable  #20 = Utf8               test2  #21 = Utf8               SourceFile  #22 = Utf8               Test.java  #23 = NameAndType        #7:#8          // "
     
      ":()V  #24 = Class              #32            // java/lang/System  #25 = NameAndType        #33:#34        // out:Ljava/io/PrintStream;  #26 = Utf8               hello  #27 = Class              #35            // java/io/PrintStream  #28 = NameAndType        #36:#37        // println:(Ljava/lang/String;)V  #29 = Utf8               com/feng/demo/Test  #30 = Utf8               java/lang/Object  #31 = Utf8               java/lang/Throwable  #32 = Utf8               java/lang/System  #33 = Utf8               out  #34 = Utf8               Ljava/io/PrintStream;  #35 = Utf8               java/io/PrintStream  #36 = Utf8               println  #37 = Utf8               (Ljava/lang/String;)V{  public com.feng.demo.Test();    descriptor: ()V    flags: ACC_PUBLIC    Code:      stack=1, locals=1, args_size=1         0: aload_0         1: invokespecial #1                  // Method java/lang/Object."
      
       ":()V         4: return      LineNumberTable:        line 3: 0      LocalVariableTable:        Start  Length  Slot  Name   Signature            0       5     0  this   Lcom/feng/demo/Test;  public synchronized void test();    descriptor: ()V    flags: ACC_PUBLIC, ACC_SYNCHRONIZED    Code:      stack=2, locals=1, args_size=1         0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;         3: ldc           #3                  // String hello         5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V         8: return      LineNumberTable:        line 5: 0        line 6: 8      LocalVariableTable:        Start  Length  Slot  Name   Signature            0       9     0  this   Lcom/feng/demo/Test;  public void test3();    descriptor: ()V    flags: ACC_PUBLIC    Code:      stack=2, locals=3, args_size=1         0: aload_0         1: dup         2: astore_1         3: monitorenter         4: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;         7: ldc           #3                  // String hello         9: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V        12: aload_1        13: monitorexit        14: goto          22        17: astore_2        18: aload_1        19: monitorexit        20: aload_2        21: athrow        22: return      Exception table:         from    to  target type             4    14    17   any            17    20    17   any      LineNumberTable:        line 9: 0        line 10: 4        line 11: 12        line 12: 22      LocalVariableTable:        Start  Length  Slot  Name   Signature            0      23     0  this   Lcom/feng/demo/Test;      StackMapTable: number_of_entries = 2        frame_type = 255 /* full_frame */          offset_delta = 17          locals = [ class com/feng/demo/Test, class java/lang/Object ]          stack = [ class java/lang/Throwable ]        frame_type = 250 /* chop */          offset_delta = 4  public static synchronized void test2();    descriptor: ()V    flags: ACC_PUBLIC, ACC_STATIC, ACC_SYNCHRONIZED    Code:      stack=2, locals=0, args_size=0         0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;         3: ldc           #3                  // String hello         5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V         8: return      LineNumberTable:        line 15: 0        line 16: 8}SourceFile: "Test.java"
      
     
    
   

对应上面的代码，class字节码，比如aload_1即class虚拟机字节码指令，其实很多语言可以编译为class文件，所有Java虚拟机实际上是class虚拟机。

可以看出本质monitorenter和monitorexit指令

同步方法其实是

flags: ACC_SYNCHRONIZED，通过-v参数可以直观地看见底层实现。

实际上，锁的本质满足如下：

1、原子性:当一个线程操作共享变量的时候，其他线程不能操作。实际就是串行修改。

2、可见性:当一个线程修改一个变量的时候，其他的线程及时知道更新缓存变量（参考计算机原理CPU与内存或主存的交互设计）

3、（可选）可重入:当一个线程拿到某个锁后，当该线程在释放该锁之前，可以重复获取该锁，当然释放次数与获取次数相同，否则死锁

从上可看出synchronized 的用途

1. 对于同步方法，锁是当前实例对象。

2. 对于同步方法块，锁是Synchonized括号里的对象。

3. 对于静态同步方法，锁是当前对象的Class对象。类似 synchronized (this.class)

JDK8及以后的版本JDK对synchronized做了优化，对于写操作频繁的线程安全推荐使用，性能较强

synchronized是操作对象头的monitor实现的，所以当时用sleep和wait的时候，在sleep方法中没有释放monitor，所以锁未释放，而wait方法释放了monitor对象，释放了锁

核心原理：JMM模型操作协议原子性

monitorenter和monitorexit指令操作lock和unlock指令， 保证主内存与工作内存的一致性