volatile关键字和synchronized关键字

volatile关键字：

可以用来修饰字段（成员变量），就是告知程序任何对该变量的访问均需要从共享内存中获取，而对它的改变必须同步刷新回共享内存，它能保证所有线程对变量访问的可见性。

synchronized关键字：

可以修饰方法或以同步块的形式来进行使用，它主要确保多个线程在同一时刻，只能有一个线程处于方法或者同步块中，它保证了对变量访问的可见性和排他性。

package com.baidu.nuomi.concurrent;

/**
 * Created by sonofelice on 16/6/18.
 */
public class Synchronized {
    public static void main(String[] args) {
        synchronized (Synchronized.class){

        }
    }
    public static synchronized void m(){}
}

使用javap查看生成的class文件：

在该类的同级目录下执行javap -v Synchronized.class

得到下面的结果：

Classfile /Users/sonofelice/Downloads/zhmmself/target/classes/com/baidu/nuomi/concurrent/Synchronized.class
  Last modified 2016-6-18; size 590 bytes
  MD5 checksum 64550d9817510bcc5d531e20e814b122
  Compiled from "Synchronized.java"
public class com.baidu.nuomi.concurrent.Synchronized
  SourceFile: "Synchronized.java"
  minor version: 0
  major version: 50
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #3.#22         //  java/lang/Object."<init>":()V
   #2 = Class              #23            //  com/baidu/nuomi/concurrent/Synchronized
   #3 = Class              #24            //  java/lang/Object
   #4 = Utf8               <init>
   #5 = Utf8               ()V
   #6 = Utf8               Code
   #7 = Utf8               LineNumberTable
   #8 = Utf8               LocalVariableTable
   #9 = Utf8               this
  #10 = Utf8               Lcom/baidu/nuomi/concurrent/Synchronized;
  #11 = Utf8               main
  #12 = Utf8               ([Ljava/lang/String;)V
  #13 = Utf8               args
  #14 = Utf8               [Ljava/lang/String;
  #15 = Utf8               StackMapTable
  #16 = Class              #14            //  "[Ljava/lang/String;"
  #17 = Class              #24            //  java/lang/Object
  #18 = Class              #25            //  java/lang/Throwable
  #19 = Utf8               m
  #20 = Utf8               SourceFile
  #21 = Utf8               Synchronized.java
  #22 = NameAndType        #4:#5          //  "<init>":()V
  #23 = Utf8               com/baidu/nuomi/concurrent/Synchronized
  #24 = Utf8               java/lang/Object
  #25 = Utf8               java/lang/Throwable
{
  public com.baidu.nuomi.concurrent.Synchronized();
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 6: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
               0       5     0  this   Lcom/baidu/nuomi/concurrent/Synchronized;

  public static void main(java.lang.String[]);
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=3, args_size=1
         0: ldc_w         #2                  // class com/baidu/nuomi/concurrent/Synchronized
         3: dup
         4: astore_1
         5: monitorenter
         6: aload_1
         7: monitorexit
         8: goto          16
        11: astore_2
        12: aload_1
        13: monitorexit
        14: aload_2
        15: athrow
        16: return
      Exception table:
         from    to  target type
             6     8    11   any
            11    14    11   any
      LineNumberTable:
        line 8: 0
        line 10: 6
        line 11: 16
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
               0      17     0  args   [Ljava/lang/String;
      StackMapTable: number_of_entries = 2
           frame_type = 255 /* full_frame */
          offset_delta = 11
          locals = [ class "[Ljava/lang/String;", class java/lang/Object ]
          stack = [ class java/lang/Throwable ]
           frame_type = 250 /* chop */
          offset_delta = 4

  public static synchronized void m();
    flags: ACC_PUBLIC, ACC_STATIC, ACC_SYNCHRONIZED
    Code:
      stack=0, locals=0, args_size=0
         0: return
      LineNumberTable:
        line 12: 0
}

分析一下synchronized关键字的实现细节。

上面class信息中，对于同步块的实现使用了

monitorenter  和   monitorexit

指令。而同步方法则是依靠方法修饰符上的

ACC_SYNCHRONIZED

来完成的。

无论哪种方式，其本质是对一个对象的监视器进行获取，而这个获取的过程是排他的，也就是同一时刻只能有一个线程获取到由synchronized所保护对象的监视器。

任意一个对象都拥有自己的监视器，当这个对象由同步块或者这个对象的同步方法调用时，执行方法的线程必须先获取到该对象的监视器才能进入同步块或者同步方法，而没有获取到监视器（执行该方法）的线程将会被阻塞在同步块和同步方法的入口处，进入BLOCKED状态。