java Integer类的缓存（转）

首先看一段代码（使用JDK 5），如下：

public class Hello
{
public static void main(String[] args)
{
int a = 1000, b = 1000;
System.out.println(a == b);
Integer c = 1000, d = 1000;
System.out.println(c == d);
Integer e = 100, f = 100;
System.out.println(e == f);
}
}

输出结果：

true
false
true

The JavaLanguage 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 Integer's 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.

巴特西

java Integer类的缓存（转）

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