英文:
Why synchronized in static method and in Class get different java byte code
问题
以下是翻译好的部分：
我已经学到，静态方法和类中的 synchronized 关键字是相同的，它们都是类级别的锁。
然后我尝试编写一个示例程序：
package com.hao.thread;
public class Main {
private static int count = 10;
public synchronized static void m1() {
--count;
System.out.println(Thread.currentThread().getName() + " count = " + count);
}
public static void m2() {
synchronized (Main.class) {
--count;
System.out.println(Thread.currentThread().getName() + " count = " + count);
}
}
public static void main(String[] args) {
}
}
编译后，使用 jclasslib（一个 IntelliJ IDEA 插件）检查 Java 字节码，我发现 m1() 和 m2() 的字节码是不同的，m1() 方法的字节码中没有 monitorenter。
以下是图片链接，由于我声望不足，只能粘贴图片的直接链接：
我想知道为什么会出现这种情况？而且有了不同的 Java 字节码，它们如何拥有相同的行为？JVM 是否为我做了额外的处理？
英文:
I have learned that synchronized in static method and Class are the same thing, they are both class level lock.
Then I try to write a demo program:
package com.hao.thread;
public class Main {
private static int count = 10;
public synchronized static void m1() {
--count;
System.out.println(Thread.currentThread().getName() + " count = " + count);
}
public static void m2() {
synchronized (Main.class) {
--count;
System.out.println(Thread.currentThread().getName() + " count = " + count);
}
}
public static void main(String[] args) {
}
}
After compile, and check the java bytecode with jclasslib(a idea plugin), I found m1() and m2() got different java bytecode, there is no monitorenter in the bytecode of m1() method.
the pictures are follows, because I don't have enough reputation, I have to paste the direct link of image
m1()java bytecode:m2()java bytecode:
I wonder why it happen? And with the different java bytecode, how can they have the same behaviour? Is there any extra thing JVM done for me?
答案1
得分: 0
参见 JVM 规范 §17.1 - 虚拟机会看到 'synchronized' 关键字，并隐式执行 monitorenter。在 Java 字节码中，有两种获取对象锁的方式：通过 synchronized 关键字或通过 MONITORENTER 字节码，就是这么简单。
英文:
See JVM Specification §17.1 - the VM sees the 'synchronized' keyword and does the monitorenter implicitly. There are 2 ways to obtain the lock on an object in java bytecode: Either via a synchronized keyword, or via a MONITORENTER bytecode, it's as simple as that.
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