Java Integer类的缓存

The Java Language Specification, 3rd Edition 写道：

为了节省内存，对于下列包装对象的两个实例，当它们的基本值相同时，他们总是==： 

 Boolean 

 Byte 

 Character, \u0000 - \u007f(7f是十进制的127) 

 Integer, -128 — 127

查看jdk源码，如下：

 /** 

 * 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. During VM initialization the 

 * getAndRemoveCacheProperties method may be used to get and remove any system 

 * properites that configure the cache size. At this time, the size of the 

 * cache may be controlled by the vm option -XX:AutoBoxCacheMax= size . 

 // value of java.lang.Integer.IntegerCache.high property (obtained during VM init) 

 private static String integerCacheHighPropValue; 

 static void getAndRemoveCacheProperties() { 

 if (!sun.misc.VM.isBooted()) { 

 Properties props = System.getProperties(); 

 integerCacheHighPropValue = 

 (String)props.remove("java.lang.Integer.IntegerCache.high"); 

 if (integerCacheHighPropValue != null) 

 System.setProperties(props); // remove from system props 

 private static class IntegerCache { 

 static final int high; 

 static final Integer cache[]; 

 static { 

 final int low = -128; 

 // high value may be configured by property 

 int h = 127; 

 if (integerCacheHighPropValue != null) { 

 // Use Long.decode here to avoid invoking methods that 

 // require Integers autoboxing cache to be initialized 

 int i = Long.decode(integerCacheHighPropValue).intValue(); 

 i = Math.max(i, 127); 

 // Maximum array size is Integer.MAX_VALUE 

 h = Math.min(i, Integer.MAX_VALUE - -low); 

 high = h; 

 cache = new Integer[(high - low) + 1]; 

 int j = low; 

 for(int k = 0; k cache.length; k++) //缓存区间数据 

 cache[k] = new Integer(j++); 

 private IntegerCache() {} 

 /** 

 * Returns a tt Integer /tt instance representing the specified 

 * tt int /tt value. 

 * If a new tt Integer /tt instance is not required, this method 

 * should generally be used in preference to the constructor 

 * {@link #Integer(int)}, as this method is likely to yield 

 * significantly better space and time performance by caching 

 * frequently requested values. 

 * @param i an code int /code value. 

 * @return a tt Integer /tt instance representing tt i /tt . 

 * @since 1.5 

 public static Integer valueOf(int i) { 

 if(i = -128 i = IntegerCache.high) 

 return IntegerCache.cache[i + 128]; 

 else 

 return new Integer(i); 

 }

这儿的IntegerCache有一个静态的Integer数组，在类加载时就将-128 到 127 的Integer对象创建了，并保存在cache数组中，一旦程序调用valueOf 方法，如果i的值是在-128 到 127 之间就直接在cache缓存数组中去取Integer对象。

再看其它的包装器：

Boolean：(全部缓存)
Byte：(全部缓存)
Character( = 127缓存)
Short(-128 — 127缓存)
Long(-128 — 127缓存)
Float(没有缓存)
Doulbe(没有缓存)

同样，对于垃圾回收器来说：

Integer i = 100; 

i = null;//will not make any object available for GC at all.

这里的代码不会有对象符合垃圾回收器的条件，这儿的i虽然被赋予null，但它之前指向的是cache中的Integer对象，而cache没有被赋null，所以Integer(100)这个对象还是存在。

而如果i大于127或小于-128则它所指向的对象将符合垃圾回收的条件：

Integer i = 10000; 

i = null;//will make the newly created Integer object available for GC.

转载地址： http://blog.csdn.net/xiaohai0504/article/details/6885137