Integer a=127,Integer b=127，a==b为true还是false？

原创已于 2022-05-17 17:03:33 修改 · 780 阅读

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于 2022-05-17 16:47:53 首次发布

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本文详细解析Integer类的自动装箱过程，说明了127与128在常量池中的不同处理方式，以及IntegerCache静态内部类的作用和缓存策略。

True，JVM会自动维护5种基本数据类型的常量池，int常量池中初始化-128到127的范围，所以当为Integer a = 127时，在自动装箱过程中是取自常量池中的数值，而当Integer a=128时，128不在常量池范围内，所以在自动装箱过程中需new 128，所以地址不一样。

以下是Integer的静态内部类

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 {
                    h = Math.max(parseInt(integerCacheHighPropValue), 127);
                    // Maximum array size is Integer.MAX_VALUE
                    h = Math.min(h, 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
            CDS.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 i = 0; i < c.length; i++) {
                    c[i] = new Integer(j++);
                }
                archivedCache = c;
            }
            cache = archivedCache;
            // range [-128, 127] must be interned (JLS7 5.1.7)
            assert IntegerCache.high >= 127;
        }

        private IntegerCache() {}
    }

VM.getSavedProperty(“java.lang.Integer.IntegerCache.high”);表示缓存范围是可以被修改的
而Byte，Short，Long 有固定范围: -128 到 127，对于 Character, 范围是 0 到 127。

LongCache

private static class LongCache {
        private LongCache() {}

        static final Long[] cache;
        static Long[] archivedCache;

        static {
            int size = -(-128) + 127 + 1;

            // Load and use the archived cache if it exists
            CDS.initializeFromArchive(LongCache.class);
            if (archivedCache == null || archivedCache.length != size) {
                Long[] c = new Long[size];
                long value = -128;
                for(int i = 0; i < size; i++) {
                    c[i] = new Long(value++);
                }
                archivedCache = c;
            }
            cache = archivedCache;
        }
    }

CharacterCache

private static class CharacterCache {
        private CharacterCache(){}

        static final Character[] cache;
        static Character[] archivedCache;

        static {
            int size = 127 + 1;

            // Load and use the archived cache if it exists
            CDS.initializeFromArchive(CharacterCache.class);
            if (archivedCache == null || archivedCache.length != size) {
                Character[] c = new Character[size];
                for (int i = 0; i < size; i++) {
                    c[i] = new Character((char) i);
                }
                archivedCache = c;
            }
            cache = archivedCache;
        }
    }

ShortCache

private static class ShortCache {
        private ShortCache() {}

        static final Short[] cache;
        static Short[] archivedCache;

        static {
            int size = -(-128) + 127 + 1;

            // Load and use the archived cache if it exists
            CDS.initializeFromArchive(ShortCache.class);
            if (archivedCache == null || archivedCache.length != size) {
                Short[] c = new Short[size];
                short value = -128;
                for(int i = 0; i < size; i++) {
                    c[i] = new Short(value++);
                }
                archivedCache = c;
            }
            cache = archivedCache;
        }
    }

ByteCache

private static class ByteCache {
        private ByteCache() {}

        static final Byte[] cache;
        static Byte[] archivedCache;

        static {
            final int size = -(-128) + 127 + 1;

            // Load and use the archived cache if it exists
            CDS.initializeFromArchive(ByteCache.class);
            if (archivedCache == null || archivedCache.length != size) {
                Byte[] c = new Byte[size];
                byte value = (byte)-128;
                for(int i = 0; i < size; i++) {
                    c[i] = new Byte(value++);
                }
                archivedCache = c;
            }
            cache = archivedCache;
        }
    }