java Integer类的缓存(转)
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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 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 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.
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