0x01：synchronized的基本語法
修飾實例方法，作用於當前實例加鎖，進入同步代碼前要獲得當前實例的鎖靜態方法，作用於當前類對象加鎖，進入同步代碼前要獲得當前類對象的鎖修飾代碼塊，指定加鎖對象，對給定對象加鎖，進入同步代碼庫前要獲得給定對象的鎖。
0x02：代碼分析synchronized
<code>package com.lesson8;
public class SynchronizedDemo {
public Object lock = new Object();
public synchronized void syncCommonMethod(){
System.out.println("==syncCommonMethod==");
}
public static synchronized void syncStaticMethod(){
System.out.println("==syncStaticMethod==");
}
public void syncBlockCode(){
synchronized (lock) {
System.out.println("==syncBlockCode==");
}
}
}/<code>
代碼中包三個方法，分別是synchronized修飾實例方法、synchronized靜態方法和synchronized修飾代碼塊。使用命令：
javap -v SynchronizedDemo.class
得到如下彙編指令：
<code>Classfile /D:/jmeterws/xml/com-lesson8/target/classes/com/lesson8/SynchronizedDemo.class
Last modified 2020-4-5; size 926 bytes
MD5 checksum f23cff99c70b8a401db1cc3bd74538ec
Compiled from "SynchronizedDemo.java"
public class com.lesson8.SynchronizedDemo
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Class #2 // com/lesson8/SynchronizedDemo
#2 = Utf8 com/lesson8/SynchronizedDemo
#3 = Class #4 // java/lang/Object
#4 = Utf8 java/lang/Object
#5 = Utf8 lock
#6 = Utf8 Ljava/lang/Object;
#7 = Utf8 <init>
#8 = Utf8 ()V
#9 = Utf8 Code
#10 = Methodref #3.#11 // java/lang/Object."<init>":()V
#11 = NameAndType #7:#8 // "<init>":()V
#12 = Fieldref #1.#13 // com/lesson8/SynchronizedDemo.lock:Ljava/lang/Object;
#13 = NameAndType #5:#6 // lock:Ljava/lang/Object;
#14 = Utf8 LineNumberTable
#15 = Utf8 LocalVariableTable
#16 = Utf8 this
#17 = Utf8 Lcom/lesson8/SynchronizedDemo;
#18 = Utf8 syncCommonMethod
#19 = Fieldref #20.#22 // java/lang/System.out:Ljava/io/PrintStream;
#20 = Class #21 // java/lang/System
#21 = Utf8 java/lang/System
#22 = NameAndType #23:#24 // out:Ljava/io/PrintStream;
#23 = Utf8 out
#24 = Utf8 Ljava/io/PrintStream;
#25 = String #26 // ==syncCommonMethod==
#26 = Utf8 ==syncCommonMethod==
#27 = Methodref #28.#30 // java/io/PrintStream.println:(Ljava/lang/String;)V
#28 = Class #29 // java/io/PrintStream
#29 = Utf8 java/io/PrintStream
#30 = NameAndType #31:#32 // println:(Ljava/lang/String;)V
#31 = Utf8 println
#32 = Utf8 (Ljava/lang/String;)V
#33 = Utf8 syncStaticMethod
#34 = String #35 // ==syncStaticMethod==
#35 = Utf8 ==syncStaticMethod==
#36 = Utf8 syncBlockCode
#37 = String #38 // ==syncBlockCode==
#38 = Utf8 ==syncBlockCode==
#39 = Utf8 StackMapTable
#40 = Class #41 // java/lang/Throwable
#41 = Utf8 java/lang/Throwable
#42 = Utf8 SourceFile
#43 = Utf8 SynchronizedDemo.java
{
public java.lang.Object lock;
descriptor: Ljava/lang/Object;
flags: ACC_PUBLIC
public com.lesson8.SynchronizedDemo();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=3, locals=1, args_size=1
0: aload_0
1: invokespecial #10 // Method java/lang/Object."<init>":()V
4: aload_0
5: new #3 // class java/lang/Object
8: dup
9: invokespecial #10 // Method java/lang/Object."<init>":()V
12: putfield #12 // Field lock:Ljava/lang/Object;
15: return
LineNumberTable:
line 3: 0
line 5: 4
line 3: 15
LocalVariableTable:
Start Length Slot Name Signature
0 16 0 this Lcom/lesson8/SynchronizedDemo;
public synchronized void syncCommonMethod();
descriptor: ()V
flags: ACC_PUBLIC, ACC_SYNCHRONIZED
Code:
stack=2, locals=1, args_size=1
0: getstatic #19 // Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #25 // String ==syncCommonMethod==
5: invokevirtual #27 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
LineNumberTable:
line 8: 0
line 9: 8
LocalVariableTable:
Start Length Slot Name Signature
0 9 0 this Lcom/lesson8/SynchronizedDemo;
public static synchronized void syncStaticMethod();
descriptor: ()V
flags: ACC_PUBLIC, ACC_STATIC, ACC_SYNCHRONIZED
Code:
stack=2, locals=0, args_size=0
0: getstatic #19 // Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #34 // String ==syncStaticMethod==
5: invokevirtual #27 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
LineNumberTable:
line 12: 0
line 13: 8
LocalVariableTable:
Start Length Slot Name Signature
public void syncBlockCode();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=2, locals=2, args_size=1
0: aload_0
1: getfield #12 // Field lock:Ljava/lang/Object;
4: dup
5: astore_1
6: monitorenter
7: getstatic #19 // Field java/lang/System.out:Ljava/io/PrintStream;
10: ldc #37 // String ==syncBlockCode==
12: invokevirtual #27 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
15: aload_1
16: monitorexit
17: goto 23
20: aload_1
21: monitorexit
22: athrow
23: return
Exception table:
from to target type
7 17 20 any
20 22 20 any
LineNumberTable:
line 16: 0
line 17: 7
line 16: 15
line 19: 23
LocalVariableTable:
Start Length Slot Name Signature
0 24 0 this Lcom/lesson8/SynchronizedDemo;
StackMapTable: number_of_entries = 2
frame_type = 255 /* full_frame */
offset_delta = 20
locals = [ class com/lesson8/SynchronizedDemo, class java/lang/Object ]
stack = [ class java/lang/Throwable ]
frame_type = 250 /* chop */
offset_delta = 2
}
SourceFile: "SynchronizedDemo.java"/<init>/<init>/<init>/<init>/<init>/<code>
分析如上代碼發現synchronized修飾實例方法、synchronized靜態方法與synchronized修飾代碼塊不一樣，synchronized修飾實例方法和synchronized靜態方法一樣
synchronized修飾代碼塊：
涉及兩條指令：
- monitorenter：每個對象有一個監視器鎖（monitor）。當monitor被佔用時就會處於鎖定狀態，線程執行monitorenter指令時嘗試獲取monitor的所有權，過程如下：
如果monitor的進入數為0，則該線程進入monitor，然後將進入數設置為1，該線程即為monitor的所有者。
如果線程已經佔有該monitor，只是重新進入，則進入monitor的進入數加1。
如果其他線程已經佔用了monitor，則該線程進入阻塞狀態，直到monitor的進入數為0，再重新嘗試獲取monitor的所有權。
- monitorexit：執行monitorexit的線程必須是objectref所對應的monitor的所有者。
指令執行時，monitor的進入數減1，如果減1後進入數為0，那線程退出monitor，不再是這個monitor的所有者。其他被這個monitor阻塞的線程可以嘗試去獲取這個monitor 的所有權。
synchronized修飾實例方法：
synchronized靜態方法：
從反編譯的結果來看，方法的同步並沒有通過指令monitorenter和monitorexit來完成（理論上其實也可以通過這兩條指令來實現）。相對於普通方法，其常量池中多了ACC_SYNCHRONIZED標示符。
JVM就是根據該標示符來實現方法的同步的：當方法被調用時，調用指令將會檢查方法的 ACC_SYNCHRONIZED 訪問標誌是否被設置，如果設置了，執行線程將先獲取monitor，獲取成功之後才能執行方法體，方法執行完後再釋放monitor。在方法執行期間，其他任何線程都無法再獲得同一個monitor對象。其實本質上沒有區別，只是方法的同步是一種隱式的方式來實現，無需通過字節碼來完成。
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