解密java对象的地址及大小:
操作系统:
windows 64
java运行环境:
java version "1.8.0_40"
Java(TM) SE Runtime Environment (build 1.8.0_40-b25)
Java HotSpot(TM) 64-Bit Server VM (build 25.40-b25, mixed mode)
main启动参数
vm options:-XX:-UseCompressedOops 关闭指针压缩
import sun.misc.Unsafe;
import java.lang.management.ManagementFactory;
import java.lang.reflect.Field;
/**
* @author paul
* @description
* @date 2018/11/5
*/
public class Test2 {
public int a;
public int b;
public int c;
public Object o1;
public Object o2 = new TestObjectSize();
public Object o3;
public int[] int1;
public int[] int2;
public int[] int3;
public char char1;
public char char2;
public char char3;
public static void main(String[] args) throws Exception {
String name = ManagementFactory.getRuntimeMXBean().getName();
System.out.println(name);
//操作系统:windows 64位
//关闭指针压缩-XX:-UseCompressedOops
Unsafe unsafeInstance = getUnsafeInstance();
{
//普通对象头大小:16Byte.
// 8:Mark Word + 8:Class Pointer Class对象（其对应的元数据对象）的内存地址
long testObjectSizeOffset = unsafeInstance.objectFieldOffset(TestObjectSize.class.getDeclaredField("flag")); //16
//testObjectSizeOffset为字段flag对于对象TestObjectSize的偏向地址,则说明对象头占用了16字节
int flag = unsafeInstance.getInt(new TestObjectSize(), testObjectSizeOffset);//100
//字段flag为基础类型int占用4字节,则getInt取到flag的值100
}
{
//数组对象头大小:24Byte:(8+8+4)
// 8:Mark Word + 8:Class Pointer Class对象（其对应的元数据对象）的内存地址 + 4:数组长度
int[] ints = new int[200];
long IntArrayOffset = unsafeInstance.arrayBaseOffset(int[].class);//24 数组对象头的大小
int intsCount = unsafeInstance.getInt(ints, 16L);//200 ints数组的大小
}
//注意在对象内存分布时,基础类型和引用类型(8Byte:为对象的地址)会在内存中分开,如果所有基础类型的size不是8的倍数则对齐填充
//下面可以展示对象填充(char3从偏移量32到34,o1偏移量为40,40-34=6填充6Byte)
Test2 test2 = new Test2();
{
long aOffset = unsafeInstance.objectFieldOffset(Test2.class.getDeclaredField("a"));//16
long bOffset = unsafeInstance.objectFieldOffset(Test2.class.getDeclaredField("b"));//20
long cOffset = unsafeInstance.objectFieldOffset(Test2.class.getDeclaredField("c"));//24
long o1Offset = unsafeInstance.objectFieldOffset(Test2.class.getDeclaredField("o1"));//40
long o2Offset = unsafeInstance.objectFieldOffset(Test2.class.getDeclaredField("o2"));//48
long o3Offset = unsafeInstance.objectFieldOffset(Test2.class.getDeclaredField("o3"));//56
long int1Offset = unsafeInstance.objectFieldOffset(Test2.class.getDeclaredField("int1"));//64
long int2Offset = unsafeInstance.objectFieldOffset(Test2.class.getDeclaredField("int2"));//72
long int3Offset = unsafeInstance.objectFieldOffset(Test2.class.getDeclaredField("int3"));//80
long char1Offset = unsafeInstance.objectFieldOffset(Test2.class.getDeclaredField("char1"));//28
long char2Offset = unsafeInstance.objectFieldOffset(Test2.class.getDeclaredField("char2"));//30
long char3Offset = unsafeInstance.objectFieldOffset(Test2.class.getDeclaredField("char3"));//32
}
//如何获取Test2中字段o2引用对象(TestObjectSize)的flag字段:
long o2Offset = unsafeInstance.objectFieldOffset(Test2.class.getDeclaredField("o2"));//48
//os为对象引用占8字节存放对象地址
long o2Address = unsafeInstance.getLong(test2, o2Offset);//此处为Test2中o2对象的地址
//TestObjectSize为普通对象,则跳过16Byte后的4字节存放int
int unsafeInstanceInt = unsafeInstance.getInt(o2Address + 16);
assert unsafeInstanceInt == ((TestObjectSize) test2.o2).flag;
System.out.println("test2地址 :" + getObjectAddress(test2));
System.out.println("test2.o2地址:" + getObjectAddress(test2.o2));
System.out.println("test2地址 :" + getObjectAddress(test2));
System.out.println("test2.o2地址:" + getObjectAddress(test2.o2));
System.out.println("test2地址 mark init:" + getObjectMarkValue(test2));
synchronized (test2) {
System.out.println("test2地址 mark lock:" + getObjectMarkValue(test2));
}
System.out.println("...");
}
public static Unsafe getUnsafeInstance() throws Exception {
Field theUnsafeInstance = Unsafe.class.getDeclaredField("theUnsafe");
theUnsafeInstance.setAccessible(true);
return (Unsafe) theUnsafeInstance.get(Unsafe.class);
}
public static class TestObjectSize {
public int flag = 100;
}
/**
* 获取目标对象头mark值
*
* @param o 目标对象
* @return 目标对象地址
*/
public static Long getObjectMarkValue(Object o) {
try {
Unsafe unsafeInstance = getUnsafeInstance();
return unsafeInstance.getLong(o, 0L);
} catch (Exception e) {
e.printStackTrace();
}
return null;
}
/**
* 获取目标对象头类型对象地址
*
* @param o 目标对象
* @return 类型对象地址
*/
public static Long getObjectClassValue(Object o) {
try {
Unsafe unsafeInstance = getUnsafeInstance();
return unsafeInstance.getLong(o, 8L);
} catch (Exception e) {
e.printStackTrace();
}
return null;
}
/**
* 获取目标对象的内存地址 10进制
*
* @param o 目标对象
* @return 目标对象地址
*/
public static Long getObjectAddress(Object o) {
ObjectAddressHolder objectAddressHolder = new ObjectAddressHolder();
objectAddressHolder.object = o;
try {
Unsafe unsafeInstance = getUnsafeInstance();
long objectFieldOffset = unsafeInstance.objectFieldOffset(ObjectAddressHolder.class.getDeclaredField("object"));
long objectAddress = unsafeInstance.getLong(objectAddressHolder, objectFieldOffset);
objectAddressHolder.object = null;
objectAddressHolder = null;
return objectAddress;
} catch (Exception e) {
e.printStackTrace();
}
return null;
}
public static class ObjectAddressHolder {
public Object object;
}
}
一个对象的大小等于:对象头+属性占用空间（基础类型+引用类型 8Byte）
关闭指针压缩:Test2大小=16(对象头) + 4(int)3 + 8(Object)3 + 8(int[])3 + 2(char)3
验证方式:jdk自带工具:java -classpath "%JAVA_HOME%/lib/sa-jdi.jar" sun.jvm.hotspot.HSDB
或者使用:使用Instrumentation查看对象的大小
