看一段代码
public static void main(String[] args) {
Integer a = 127;
Integer b = 127;
Integer c = 128;
Integer d = 128;
System.out.println(a == b);
System.out.println(c == d);
}
猜一猜结果,看看结果是不是你想的那样?
true
false
cache % javac Test.java
cache % java Test
true
false
cache % javap -c Test
Compiled from "Test.java"
public class Test {
public Test();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
public static void main(java.lang.String[]);
Code:
0: bipush 127
2: invokestatic #7 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
5: astore_1
6: bipush 127
8: invokestatic #7 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
11: astore_2
12: sipush 128
15: invokestatic #7 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
18: astore_3
19: sipush 128
22: invokestatic #7 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
25: astore 4
27: getstatic #13 // Field java/lang/System.out:Ljava/io/PrintStream;
30: aload_1
31: aload_2
32: if_acmpne 39
35: iconst_1
36: goto 40
39: iconst_0
40: invokevirtual #19 // Method java/io/PrintStream.println:(Z)V
43: getstatic #13 // Field java/lang/System.out:Ljava/io/PrintStream;
46: aload_3
47: aload 4
49: if_acmpne 56
52: iconst_1
53: goto 57
56: iconst_0
57: invokevirtual #19 // Method java/io/PrintStream.println:(Z)V
60: return
}
从字节码指令来看并没有太多差别
30: aload_1
31: aload_2
32: if_acmpne 39
35: iconst_1
都是两步,加载判断写入
这里就有点看头了,IntegerCache.low
和IntegerCache.high
@HotSpotIntrinsicCandidate
public static Integer valueOf(int i) {
if (i >= IntegerCache.low && i <= IntegerCache.high)
return IntegerCache.cache[i + (-IntegerCache.low)];
return new Integer(i);
}
数组缓存的最低最高
可以看到系统配置的属性,最高会和127做比较否则127就是最高
可以看到到了127地址都不一样了
从-128到127
片段2
看到初始化,这下子明白了吗?128直接创建对象了。
/**
* Cache to support the object identity semantics of autoboxing for values between
* -128 and 127 (inclusive) as required by JLS.
*
* The cache is initialized on first usage. The size of the cache
* may be controlled by the {@code -XX:AutoBoxCacheMax=<size>} option.
* During VM initialization, java.lang.Integer.IntegerCache.high property
* may be set and saved in the private system properties in the
* jdk.internal.misc.VM class.
*/
private static class IntegerCache {
static final int low = -128;
static final int high;
static final Integer[] cache;
static Integer[] archivedCache;
static {
// high value may be configured by property
int h = 127;
String integerCacheHighPropValue =
VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
if (integerCacheHighPropValue != null) {
try {
int i = parseInt(integerCacheHighPropValue);
i = Math.max(i, 127);
// Maximum array size is Integer.MAX_VALUE
h = Math.min(i, Integer.MAX_VALUE - (-low) -1);
} catch( NumberFormatException nfe) {
// If the property cannot be parsed into an int, ignore it.
}
}
high = h;
// Load IntegerCache.archivedCache from archive, if possible
VM.initializeFromArchive(IntegerCache.class);
int size = (high - low) + 1;
// Use the archived cache if it exists and is large enough
if (archivedCache == null || size > archivedCache.length) {
Integer[] c = new Integer[size];
int j = low;
for(int k = 0; k < c.length; k++)
c[k] = new Integer(j++);
archivedCache = c;
}
cache = archivedCache;
// range [-128, 127] must be interned (JLS7 5.1.7)
assert IntegerCache.high >= 127;
}
private IntegerCache() {}
}
只需要关注重点代码:for(int k = 0; k < c.length; k++)
c[k] = new Integer(j++);
这就是初始化的逻辑,看到这里就明白了吧
这代码看着还算简单的了,会调试就好分析。多多看看源码欣赏别人写代码的规范,也是一种享受。
原创声明:本文系作者授权腾讯云开发者社区发表,未经许可,不得转载。
如有侵权,请联系 cloudcommunity@tencent.com 删除。
原创声明:本文系作者授权腾讯云开发者社区发表,未经许可,不得转载。
如有侵权,请联系 cloudcommunity@tencent.com 删除。