成神之路-基础篇3-拆装箱与Integer

Integer源码分析与自动装箱的缓存机制

一、自动装箱实现原理

基本类型与包装器类型
8种基本类型均有对应的包装器类型
装箱与拆箱场景
1. 初始化
2. 容器
3. 返回值
4. 运算
实现原理

public class TestBoxing {
    public static void main(String[] args) {
        Integer i = 1;
        int j = i;
        System.out.println(i);
        System.out.println(j);
    }
}

反编译后：

Compiled from "TestBoxing.java"
public class javase.TestBoxing {
  public javase.TestBoxing();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: iconst_1
       1: invokestatic  #2                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
       4: astore_1
       5: aload_1
       6: invokevirtual #3                  // Method java/lang/Integer.intValue:()I
       9: istore_2
      10: getstatic     #4                  // Field java/lang/System.out:Ljava/io/PrintStream;
      13: aload_1
      14: invokevirtual #5                  // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
      17: getstatic     #4                  // Field java/lang/System.out:Ljava/io/PrintStream;
      20: iload_2
      21: invokevirtual #6                  // Method java/io/PrintStream.println:(I)V
      24: return
}

即：

装箱：valueOf() 返回值对应的包装器对象
拆箱：intValue() 返回包装器具体的值

注意点
包装器为对象，比较需要用equals方法

二、奇怪的现象

public class TestInteger {
    public static void main(String[] args) {
        Integer i1 = 10;
        Integer i2 = 10;
        Integer i3 = 1000;
        Integer i4 = 1000;
        System.out.println(i1 == i2);
        System.out.println(i3 == i4);
    }
}

输出：

true
false

三、源码分析

valueOf：正常对应-128到127的，直接使用cache中已创建好的对象，减小频繁创建对象开销

    public static Integer valueOf(int i) {
        if (i >= IntegerCache.low && i <= IntegerCache.high)
            return IntegerCache.cache[i + (-IntegerCache.low)];
        return new Integer(i);
    }

IntegerCache：创建一定范围的cache

    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() {}
    }

cache内比较可以用==，超出范围必须用eauqls方法

最后编辑于：2020.06.09 14:57:00