How to realize synchronized

synchronized It's the code used by many friends, and it's different from keywords , Let's take a look synchronized How is it implemented at the bytecode level

On the first code

public class Main {

    public static void main(String[] args) throws InterruptedException {}

    public  void m() {
        Object object = new Object();
        synchronized (object) {
        }
    }
}

This code actually has no task logic , There's one m Method , And there is an empty in the method synchronized Code block

Let's use idea Look at bytecode with the tools in view->show Bytecode

  public m()V
    TRYCATCHBLOCK L0 L1 L2 null
    TRYCATCHBLOCK L2 L3 L2 null
   L4
    LINENUMBER 12 L4
    NEW java/lang/Object
    DUP
    INVOKESPECIAL java/lang/Object.<init> ()V
    ASTORE 1
   L5
    LINENUMBER 13 L5
    ALOAD 1
    DUP
    ASTORE 2
    MONITORENTER
   L0
    LINENUMBER 14 L0
    ALOAD 2
    MONITOREXIT
   L1
    GOTO L6
   L2
   FRAME FULL [org/example/Main java/lang/Object java/lang/Object] [java/lang/Throwable]
    ASTORE 3
    ALOAD 2
    MONITOREXIT
   L3
    ALOAD 3
    ATHROW
   L6
    LINENUMBER 15 L6
   FRAME CHOP 1
    RETURN
   L7
    LOCALVARIABLE this Lorg/example/Main; L4 L7 0
    LOCALVARIABLE object Ljava/lang/Object; L5 L7 1
    MAXSTACK = 2
    MAXLOCALS = 4

Let's focus on the following code

  MONITORENTER
   L0
    LINENUMBER 13 L0
    ALOAD 1
 MONITOREXIT

Here it is synchronized In the implementation of bytecode level, let's see the removal synchronized Bytecode after block

  public m()V
   L0
    LINENUMBER 12 L0
    NEW java/lang/Object
    DUP
    INVOKESPECIAL java/lang/Object.<init> ()V
    ASTORE 1
   L1
    LINENUMBER 13 L1
    RETURN
   L2
    LOCALVARIABLE this Lorg/example/Main; L0 L2 0
    LOCALVARIABLE object Ljava/lang/Object; L1 L2 1
    MAXSTACK = 2
    MAXLOCALS = 2

  It is obvious from the two byte codes MONITORENTER and  MONITOREXIT Removed , From here we can know synchronized Implementation at bytecode level

synchronized The synchronization code block is implemented at the bytecode level as follows

MONITORENTER
...

 Our synchronization code block 
...

MONITOREXIT