之前在网上看到一个面试题目：synchronzied 最好的锁对象是什么
当时第一反应是 new Object，但是后来看到这篇文章：Object vs byte[0] as lock
大意就是 new byte[0]作为锁对象会更好，能够减少字节码操作的次数
下面来验证一下
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| public class ObjectSynchronizedDemo {
private final Object obj = new Object();
}
public class ArraySynchronizedDemo {
private final byte[] lockObj = new byte[0];
}
|
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| ObjectSynchronizedDemo.class
{
public com.aias.learn.test.ObjectSynchronizedDemo();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=3, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: aload_0
5: new #2 // class java/lang/Object
8: dup
9: invokespecial #1 // Method java/lang/Object."<init>":()V
12: putfield #3 // Field obj:Ljava/lang/Object;
15: return
LineNumberTable:
line 7: 0
line 8: 4
LocalVariableTable:
Start Length Slot Name Signature
0 16 0 this Lcom/aias/learn/test/ObjectSynchronizedDemo;
}
ArraySynchronizedDemo.class
{
public com.aias.learn.test.ArraySynchronizedDemo();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=2, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: aload_0
5: iconst_0
6: newarray byte
8: putfield #2 // Field lockObj:[B
11: return
LineNumberTable:
line 7: 0
line 8: 4
LocalVariableTable:
Start Length Slot Name Signature
0 12 0 this Lcom/aias/learn/test/ArraySynchronizedDemo;
}
|
可以看出，new byte[0]确实比 new Object() 少 4 条字节码操作。
再计算一下内存占用情况：
new Object()内存占用：
Mark Word(8 字节)+klass pointer(开启指针压缩，4 字节)+对齐填充(4 字节) = 16 字节
new byte[0]内存占用：
Mark Word(8 字节)+klass pointer(开启指针压缩，4 字节)+数组长度(int，4 字节) = 16 字节
所以两种锁对象占用内存是相等的。