java对象的内存地址与hashcode值关系

妖狐艹你老母 2023-02-09 14:12 3阅读 0赞

为了比较java对象的内存地址与hashcode的关系，必须想办法获取这两个值

[Java][]不能直接访问[操作系统][Link 1]底层，而是通过本地方法来访问。Unsafe类提供了硬件级别的原子操作,在java中内存中的对象地址是可变的，所以获得的内存地址有可能会变化。要获得内存地址也只能通过Unsafe的方法来获得,具体下看的代码

package com.test.democlass;
    
    import sun.misc.Unsafe;
    
    import java.lang.reflect.Field;
    
    /**
     * Created by 15002 on 2020/5/19.
     */
    public class AddresserTest {
            private static Unsafe unsafe;
            static {
                try {
                    Field field = Unsafe.class.getDeclaredField("theUnsafe");
                    field.setAccessible(true);
                    unsafe = (Unsafe) field.get(null);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
    
            public static long addressOf(Object o) throws Exception {
                Object[] array = new Object[] { o };
                long baseOffset = unsafe.arrayBaseOffset(Object[].class);
                //arrayBaseOffset方法是一个本地方法，可以获取数组第一个元素的偏移地址
                int addressSize = unsafe.addressSize();
                long objectAddress;
                switch (addressSize) {
                    case 4:
                        objectAddress = unsafe.getInt(array, baseOffset);
                        //getInt方法获取对象中offset偏移地址对应的int型field的值
                        break;
                    case 8:
                        objectAddress = unsafe.getLong(array, baseOffset);
                        //getLong方法获取对象中offset偏移地址对应的long型field的值
                        break;
                    default:
                        throw new Error("unsupported address size: " + addressSize);
                }
                return (objectAddress);
            }
    
            public static void main(String... args) throws Exception {
                String str ="Hello world";
                Object mine = str.toCharArray(); //先把字符串转化为数组对象
                long address = addressOf(mine);
                System.out.println("str的内存地址: " + address);
                System.out.println("str的hashcode: " + str.hashCode());
                // Verify address works - should see the characters in the array in the output
                printBytes(address, 27);
            }
    
            public static void printBytes(long objectAddress, int num) {
                for (long i = 0; i < num; i++) {
                    int cur = unsafe.getByte(objectAddress + i);
                    System.out.print((char) cur);
                }
                System.out.println();
            }
        }

运行结果

str的内存地址: 3804983656
    str的hashcode: -832992604

所以内存地与hashcode完全不同

[Java]: http://lib.csdn.net/base/javase
[Link 1]: http://lib.csdn.net/base/operatingsystem