java中我们都知道synchronized关键字是用来加锁，在多线程并发情况下实现同步的，那它底层是怎么实现的呢？
代码清单1-1
public class SyncTest {
public void test(){
synchronized(this){
System.out.println("-------call test() method---------");
}
}
}
代码清单1-1的字节码（通过javap -c SyncTest 查看字节码)
public class SyncTest {
public SyncTest();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":
()V
4: return
public void test();
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 -------call test() method---------
9: invokevirtual #4 // Method java/io/PrintStream.printl
n:(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
}
注意：synchronized关键词是通过的monitorenter、monitorexit这两条指令来实现同步锁的。
代码清单1-2
public class SyncTest {
public synchronized void test(){
System.out.println("-------call test() method---------");
}
}
代码清单1-2的字节码（通过javap -c SyncTest 查看字节码)
Compiled from "SyncTest.java"
public class SyncTest {
public SyncTest();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
public synchronized void test();
Code:
0: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #3 // String -------call test() method---------
5: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
}
没有什么特别的指令，那通过javap -verbose可以看到更加清楚的信息
代码清单1-2的字节码（通过javap -verbose SyncTest 查看字节码)
Classfile /D:/test/SyncTest.class从反编译的结果来看，当synchronized关键词加在方法上时并没有通过指令monitorenter和monitorexit来完成（理论上其实也可以通过这两条指令来实现），不过相对于普通方法，其常量池中多了ACC_SYNCHRONIZED标示符。JVM就是根据该标示符来实现方法的同步的：当方法调用时，调用指令将会检查方法的 ACC_SYNCHRONIZED 访问标志是否被设置，如果设置了，执行线程将先获取monitor，获取成功之后才能执行方法体，方法执行完后再释放monitor。在方法执行期间，其他任何线程都无法再获得同一个monitor对象。 其实本质上没有区别，只是方法的同步是一种隐式的方式来实现，无需通过字节码来完成。
Last modified 2017-1-19; size 419 bytes
MD5 checksum 4c415338276c1a53414a2a3890bbd654
Compiled from "SyncTest.java"
public class SyncTest
SourceFile: "SyncTest.java"
minor version: 0
major version: 51
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #6.#14 // java/lang/Object."<init>":()V
#2 = Fieldref #15.#16 // java/lang/System.out:Ljava/io/PrintStream;
#3 = String #17 // -------call test() method---------
#4 = Methodref #18.#19 // java/io/PrintStream.println:(Ljava/lang/String;)V
#5 = Class #20 // SyncTest
#6 = Class #21 // java/lang/Object
#7 = Utf8 <init>
#8 = Utf8 ()V
#9 = Utf8 Code
#10 = Utf8 LineNumberTable
#11 = Utf8 test
#12 = Utf8 SourceFile
#13 = Utf8 SyncTest.java
#14 = NameAndType #7:#8 // "<init>":()V
#15 = Class #22 // java/lang/System
#16 = NameAndType #23:#24 // out:Ljava/io/PrintStream;
#17 = Utf8 -------call test() method---------
#18 = Class #25 // java/io/PrintStream
#19 = NameAndType #26:#27 // println:(Ljava/lang/String;)V
#20 = Utf8 SyncTest
#21 = Utf8 java/lang/Object
#22 = Utf8 java/lang/System
#23 = Utf8 out
#24 = Utf8 Ljava/io/PrintStream;
#25 = Utf8 java/io/PrintStream
#26 = Utf8 println
#27 = Utf8 (Ljava/lang/String;)V
{
public SyncTest();
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 2: 0
public synchronized void test();
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 -------call test() method---------
5: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
LineNumberTable:
line 5: 0
line 6: 8
}
代码清单1-3
public class SyncTest {
public static synchronized void test(){
System.out.println("-------call test() method---------");
}
}
代码清单1-3的字节码（通过javap -verbose SyncTest 查看字节码)
Classfile /D:/test/SyncTest.class当synchronized关键词加在静态方法上面，从反编译的字节码上来看，多了一个ACC_STATIC标识。
Last modified 2017-1-19; size 419 bytes
MD5 checksum fe4b3f3196ff30f7113a035ef99f26a0
Compiled from "SyncTest.java"
public class SyncTest
SourceFile: "SyncTest.java"
minor version: 0
major version: 51
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #6.#14 // java/lang/Object."<init>":()V
#2 = Fieldref #15.#16 // java/lang/System.out:Ljava/io/PrintStream;
#3 = String #17 // -------call test() method---------
#4 = Methodref #18.#19 // java/io/PrintStream.println:(Ljava/lang/String;)V
#5 = Class #20 // SyncTest
#6 = Class #21 // java/lang/Object
#7 = Utf8 <init>
#8 = Utf8 ()V
#9 = Utf8 Code
#10 = Utf8 LineNumberTable
#11 = Utf8 test
#12 = Utf8 SourceFile
#13 = Utf8 SyncTest.java
#14 = NameAndType #7:#8 // "<init>":()V
#15 = Class #22 // java/lang/System
#16 = NameAndType #23:#24 // out:Ljava/io/PrintStream;
#17 = Utf8 -------call test() method---------
#18 = Class #25 // java/io/PrintStream
#19 = NameAndType #26:#27 // println:(Ljava/lang/String;)V
#20 = Utf8 SyncTest
#21 = Utf8 java/lang/Object
#22 = Utf8 java/lang/System
#23 = Utf8 out
#24 = Utf8 Ljava/io/PrintStream;
#25 = Utf8 java/io/PrintStream
#26 = Utf8 println
#27 = Utf8 (Ljava/lang/String;)V
{
public SyncTest();
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 2: 0
public static synchronized void test();
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 -------call test() method---------
5: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
LineNumberTable:
line 5: 0
line 6: 8
}
monitorenter ：
Each object is associated with a monitor. A monitor is locked if and only if it has an owner. The thread that executes monitorenter attempts to gain ownership of the monitor associated with objectref, as follows:
• If the entry count of the monitor associated with objectref is zero, the thread enters the monitor and sets its entry count to one. The thread is then the owner of the monitor.
• If the thread already owns the monitor associated with objectref, it reenters the monitor, incrementing its entry count.
• If another thread already owns the monitor associated with objectref, the thread blocks until the monitor's entry count is zero, then tries again to gain ownership.
这段话的大概意思为：
每个对象有一个监视器锁（monitor）。当monitor被占用时就会处于锁定状态，线程执行monitorenter指令时尝试获取monitor的所有权，过程如下：
1、如果monitor的进入数为0，则该线程进入monitor，然后将进入数设置为1，该线程即为monitor的所有者。
2、如果线程已经占有该monitor，只是重新进入，则进入monitor的进入数加1.
3.如果其他线程已经占用了monitor，则该线程进入阻塞状态，直到monitor的进入数为0，再重新尝试获取monitor的所有权。
monitorexit：
The thread that executes monitorexit must be the owner of the monitor associated with the instance referenced by objectref.
The thread decrements the entry count of the monitor associated with objectref. If as a result the value of the entry count is zero, the thread exits the monitor and is no longer its owner. Other threads that are blocking to enter the monitor are allowed to attempt to do so.
这段话的大概意思为：执行monitorexit的线程必须是objectref所对应的monitor的所有者。指令执行时，monitor的进入数减1，如果减1后进入数为0，那线程退出monitor，不再是这个monitor的所有者。其他被这个monitor阻塞的线程可以尝试去获取这个 monitor 的所有权。 通过这两段描述，我们应该能很清楚的看出Synchronized的实现原理，Synchronized的语义底层是通过一个monitor的对象来完成，其实wait/notify等方法也依赖于monitor对象，这就是为什么只有在同步的块或者方法中才能调用wait/notify等方法，否则会抛出java.lang.IllegalMonitorStateException的异常的原因。
参考资料：
http://www.cnblogs.com/paddix/p/5367116.html