自身范围缓存
boolean
boolean 的包装类型,缓存最简单,直接定义为静态常量就可以
public final class Boolean implements java.io.Serializable,
Comparable<Boolean>
{
public static final Boolean TRUE = new Boolean(true);
public static final Boolean FALSE = new Boolean(false);
public static Boolean valueOf(boolean b) {
return (b ? TRUE : FALSE);
}
}
char
ascii 码范围为 0-127, 这里只缓存ascii码范围的char
Character 静态内部类 CharacterCache
不过通过下面这段代码可以看出来，他的范围是: 0<=x<=127
public final
class Character implements java.io.Serializable, Comparable<Character> {
public static Character valueOf(char c) {
if (c <= 127) { // must cache
return CharacterCache.cache[(int)c];
}
return new Character(c);
}
private static class CharacterCache {
private CharacterCache(){}
static final Character cache[] = new Character[127 + 1];
static {
for (int i = 0; i < cache.length; i++)
cache[i] = new Character((char)i);
}
}
}
byte
byte 的范围是: -128<=x<=127
所以byte 一定是从缓存里面获取
-(-128) + 127 + 1 这一段的意思就是说 (-128~-1)+(1~127)+0 一共256个数
public final class Byte extends Number implements Comparable<Byte> {
public static Byte valueOf(byte b) {
final int offset = 128;
return ByteCache.cache[(int)b + offset];
}
private static class ByteCache {
private ByteCache(){}
static final Byte cache[] = new Byte[-(-128) + 127 + 1];
static {
for(int i = 0; i < cache.length; i++)
cache[i] = new Byte((byte)(i – 128));
}
}
}
范围为256的缓存
short
和byte 范围一样, 但是如果超过了 -128<=x<=127 这个范围,就不会从缓存中获取了
public final class Short extends Number implements Comparable<Short> {
public static Short valueOf(short s) {
final int offset = 128;
int sAsInt = s;
if (sAsInt >= -128 && sAsInt <= 127) { // must cache
return ShortCache.cache[sAsInt + offset];
}
return new Short(s);
}
private static class ShortCache {
private ShortCache(){}
static final Short cache[] = new Short[-(-128) + 127 + 1];
static {
for(int i = 0; i < cache.length; i++)
cache[i] = new Short((short)(i – 128));
}
}
}
int (可以通过参数配置上限)
这里的从缓存中获取的代码是一样的,重点是 IntegerCache 的范围是可以配置的
从源码中可以看出来 最低值一定是-128 是不可以修改的,但是上限值high 是可以修改的。
通过jvm参数: -Djava.lang.Integer.IntegerCache.high=1024 修改为1024
- 判断是否有jvm参数java.lang.Integer.IntegerCache.high
- 解析为 int 类型(解析失败就用默认值127)
- 修改最大值和最小值
4. 将 low<=x<=high的值加入到缓存
5. 验证 IntegerCache.high > 127
-
如果小于127 那么 high = 127
-
如果小于127 那么 high = 127
public final class Integer extends Number implements Comparable<Integer> {
public static Integer valueOf(int i) {
if (i >= IntegerCache.low && i <= IntegerCache.high)
return IntegerCache.cache[i + (-IntegerCache.low)];
return new Integer(i);
}
private static class IntegerCache {
static final int low = -128;
static final int high;
static final Integer cache[];
static {
// high value may be configured by property
int h = 127;
String integerCacheHighPropValue =
sun.misc.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;
cache = new Integer[(high – low) + 1];
int j = low;
for(int k = 0; k < cache.length; k++)
cache[k] = new Integer(j++);
// range [-128, 127] must be interned (JLS7 5.1.7)
assert IntegerCache.high >= 127;
}
private IntegerCache() {}
}
}
这里的 IntegerCache 无法断点,因为在进入main方法之前,就已经被其他的类加载了.
long
范围: -128<=x<=127
public final class Long extends Number implements Comparable<Long> {
public static Long valueOf(long l) {
final int offset = 128;
if (l >= -128 && l <= 127) { // will cache
return LongCache.cache[(int)l + offset];
}
return new Long(l);
}
private static class LongCache {
private LongCache(){}
static final Long cache[] = new Long[-(-128) + 127 + 1];
static {
for(int i = 0; i < cache.length; i++)
cache[i] = new Long(i – 128);
}
}
}
不被缓存
valueOf 时, 直接new 包装类型的对象,然后返回
float
public final class Float extends Number implements Comparable<Float> {
public static Float valueOf(float f) {
return new Float(f);
}
}
double
public final class Double extends Number implements Comparable<Double> {
public static Double valueOf(double d) {
return new Double(d);
}
}