Memory layout of Java Objects (ii): Use Sun.misc.Unsafe to get the offset address of the class field and the value of the Read field

In the previous article. We have listed several conclusions about calculating the size of Java objects and the use of Jol tools, and the source code of the Jol tool is interesting to look at. Now we use the Sun.misc.Unsafe in the JDK to calculate the offset address of the next field, a validation of the conclusions in the previous article, and a comparison with the Jol output results. How to get the Sun.misc.Unsafe object. Can participate in this article.

public class Vo{public int a = 0;public long B = 0;public static String c= "123";p ublic static Object d= null;public Stati c int e = 100;}

1. Get the offset address of the instance field

Gets the offset address of the instance field, and the field with the smallest offset (only next to the head) is the size of the object header System.out.println (Unsafe.objectfieldoffset (VO.class.getDeclaredField ("a "))); System.out.println (Unsafe.objectfieldoffset (VO.class.getDeclaredField ("B"));// FieldOffset as with Objectfieldoffset functions, FieldOffset is an outdated method and it is best not to use SYSTEM.OUT.PRINTLN (Unsafe.fieldoffset ( VO.class.getDeclaredField ("B")));

2. Get the head size and element size of the array

The offset address of the first element of the array, which is the number of bytes occupied by the array header int[] intarr = new Int[0]; System.out.println (Unsafe.arraybaseoffset (Intarr.getclass ()));//the size occupied by each element in the array System.out.println ( Unsafe.arrayindexscale (Intarr.getclass ()));

There are very many constants in the unsafe class that end with Base_offset, such as Array_int_base_offset, which are obtained through the Arraybaseoffset method. The Arraybaseoffset method is a local method that can get the offset address of the first element of the array. There are many constants in the unsafe class that end with Index_scale, such as Array_int_index_scale. These constant values are obtained by means of the Arrayindexscale method. Use Arraybaseoffset with Arrayindexscale to position each element in the array in memory.

3. Get the static field offset for a class

Gets the static field offset address of the class System.out.println (Unsafe.staticfieldoffset (VO.class.getDeclaredField ("C"))); System.out.println (Unsafe.staticfieldoffset (VO.class.getDeclaredField ("D")));//Gets the starting address of the static field, via the start address and offset address, You can get the value of the static field//Just the starting address of the static field, type is not long, but object type Object base1 = Unsafe.staticfieldbase (vo.class); Object base2 = Unsafe.staticfieldbase (VO.class.getDeclaredField ("D")); System.out.println (BASE1==BASE2);//true

4. Get the number of bits in the operating system

  the report the size in bytes of a native pointer.//  returns 4 or 8, which represents a 32-bit or 64-bit operating system. System.out.println (Unsafe.addresssize ());//returns 32 or 64, gets whether the operating system is 32-bit or 64-bit System.out.println (System.getproperty (" Sun.arch.data.model "));

Pass the above snippet of code. We can successfully get the offset address of each field in the class, which is consistent with the output of the Jol tool and our conclusion.

With the offset address of the field, add the object's starting place. We can get the value of the field directly through unsafe.

5. Read the value of the object instance field

Gets the property value of the instance field Vo vo = new Vo (); vo.a = 10000;long Aoffset = Unsafe.objectfieldoffset (VO.class.getDeclaredField ("a")); int VA = Unsafe.getint (vo, Aoffset); System.out.println ("va=" +va);

6. Get the property value for a static field

VO.E = 1024; Field Sfield = VO.class.getDeclaredField ("E"), Object base = Unsafe.staticfieldbase (Sfield), long offset = Unsafe.staticfieldoffset (Sfield); System.out.println (Unsafe.getint (base, offset));//1024

The ability to see the unsafe feature is very powerful, and the bit Java language provides a lower level of functionality.

Memory layout of Java Objects (ii): Use Sun.misc.Unsafe to get the offset address of the class field and the value of the Read field