两种计算Java对象大小的方法

2022-10-01 22:48:11

之前想研究一下unsafe类，碰巧在网上看到了这篇文章，觉得写得很好，就转载过来。原文出处是： http://blog.csdn.net/iter_zc/article/details/41822719

1 基础知识

普通对象的结构如下，按64位机器的长度计算 1. 对象头(_mark)， 8个字节 2. Oop指针，如果是32G内存以下的，默认开启对象指针压缩，4个字节 3. 数据区 4. Padding(内存对齐)，按照8的倍数对齐

数组对象结构是 1. 对象头(_mark)， 8个字节 2. Oop指针，如果是32G内存以下的，默认开启对象指针压缩，4个字节 3. 数组长度，4个字节 4. 数据区 5. Padding(内存对齐)，按照8的倍数对齐

清楚了对象在内存的基本布局后，咱们说两种计算Java对象大小的方法

通过java.lang.instrument.Instrumentation的getObjectSize(obj)直接获取对象的大小
通过sun.misc.Unsafe对象的objectFieldOffset(field)等方法结合反射来计算对象的大小

2 instrument的getObjectSize(obj)

先讲讲java.lang.instrument.Instrumentation.getObjectSize()的方式，这种方法得到的是Shallow Size，即遇到引用时，只计算引用的长度，不计算所引用的对象的实际大小。如果要计算所引用对象的实际大小，可以通过递归的方式去计算。

java.lang.instrument.Instrumentation的实例必须通过指定javaagent的方式才能获得，具体的步骤如下： 1. 定义一个类，提供一个premain方法: public static void premain(String agentArgs, Instrumentation instP) 2. 创建META-INF/MANIFEST.MF文件，内容是指定PreMain的类是哪个： Premain-Class: sizeof.ObjectShallowSize 3. 把这个类打成jar，然后用java -javaagent XXXX.jar XXX.main的方式执行

下面先定义一个类来获得java.lang.instrument.Instrumentation的实例,并提供了一个static的sizeOf方法对外提供Instrumentation的能力

package sizeof;  

import java.lang.instrument.Instrumentation;  

public class ObjectShallowSize {

    private static Instrumentation inst;  

    public static void premain(String agentArgs, Instrumentation instP){

        inst = instP;

    }  

    public static long sizeOf(Object obj){

        return inst.getObjectSize(obj);

    }

}

定义META-INF/MANIFEST.MF文件

Premain-Class: sizeof.ObjectShallowSize

打成jar包

cd 编译后的类和META-INF文件夹所在目录

jar cvfm java-agent-sizeof.jar META-INF/MANIFEST.MF

准备好了这个jar之后，我们可以写测试类来测试Instrumentation的getObjectSize方法了。在这之前我们先来看对象在内存中是按照什么顺序排列的

有如下这个类，字段的定义按如下顺序

private static class ObjectA {

        String str;  // 4

        int i1; // 4

        byte b1; // 1

        byte b2; // 1

        int i2;  // 4

        ObjectB obj; //4

        byte b3;  // 1

}

按照我们之前说的方法来计算一下这个对象所占大小，注意按8对齐 8(_mark) + 4(oop指针) + 4(str) + 4(i1) + 1(b1) + 1(b2) + 2(padding) + 4(i2) + 4(obj) + 1(b3) + 7(padding) = 40 ?

但事实上是这样的吗？我们来用Instrumentation的getObjectSize来计算一下先:

package test;  

import sizeof.ObjectShallowSize;  

public class SizeofWithInstrumetation {

    private static class ObjectA {

        String str;  // 4

        int i1; // 4

        byte b1; // 1

        byte b2; // 1

        int i2;  // 4

        ObjectB obj; //4

        byte b3;  // 1

    }  

    private static class ObjectB {  

    }  

    public static void main(String[] args){

        System.out.println(ObjectShallowSize.sizeOf(new ObjectA()));

    }

}

得到的结果是32！不是会按8对齐吗，b3之前的数据加起来已经是32了，多了1个b3，为33，应该对齐到40才对啊。事实上，HotSpot创建的对象的字段会先按照给定顺序排列一下,默认的顺序如下，从长到短排列，引用排最后: long/double –> int/float –> short/char –> byte/boolean –> Reference

这个顺序可以使用JVM参数: -XX:FieldsAllocationSylte=0(默认是1)来改变。

我们使用sun.misc.Unsafe对象的objectFieldOffset方法来验证一下:

Field[] fields = ObjectA.class.getDeclaredFields();

        for(Field f: fields){

            System.out.println(f.getName() + " offset: " +unsafe.objectFieldOffset(f));

        }

可以看到确实是按照从长到短，引用排最后的方式在内存中排列的。按照这种方法我们来重新计算下ObjectA创建的对象的长度:

8(_mark) + 4(oop指针) + 4(i1) + + 4(i2) + 1(b1) + 1(b2) + 1(b3) + 1(padding) + 4(str) + 4(obj) = 32 得到的结果和java.lang.instrument.Instrumentation.getObjectSize()的结果是一样的，证明我们的计算方式是正确的。

3 sun.misc.Unsafe的方式

下面说一下通过sun.misc.Unsafe对象的objectFieldOffset(field)等方法结合反射来计算对象的大小。基本的思路如下: 1. 通过反射获得一个类的Field 2. 通过Unsafe的objectFieldOffset()获得每个Field的offSet 3. 对Field按照offset排序，取得最大的offset，然后加上这个field的长度，再加上Padding对齐

上面三步就可以获得一个对象的Shallow size。可以进一步通过递归去计算所引用对象的大小，从而可以计算出一个对象所占用的实际大小。

如何获得Unsafe对象已经在这篇中聊聊序列化（二）使用sun.misc.Unsafe绕过new机制来创建Java对象说过了，可以通过反射的机制来获得.

Oop指针是4还是未压缩的8也可以通过unsafe.arrayIndexScale(Object[].class)来获得，这个方法返回一个引用所占用的长度

static {

        try {

            Field field = Unsafe.class.getDeclaredField("theUnsafe");

            field.setAccessible(true);

            unsafe = (Unsafe) field.get(null);  

            objectRefSize = unsafe.arrayIndexScale(Object[].class);

        } catch (Exception e) {

            throw new RuntimeException(e);

        }

    }

下面的源码摘自 http://java-performance.info/memory-introspection-using-sun-misc-unsafe-and-reflection/，原文中的代码在计算对象大小的时候有问题，我做了微调，并加上了内存对齐的方法，这样计算出的结果和Instrumentation的getObjectSize方法是一样的。

package test;  

import java.util.ArrayList;

import java.util.Collections;

import java.util.Comparator;

import java.util.List;  

/**

 * This class contains object info generated by ClassIntrospector tool

 */

public class ObjectInfo {

    /** Field name */

    public final String name;

    /** Field type name */

    public final String type;

    /** Field data formatted as string */

    public final String contents;

    /** Field offset from the start of parent object */

    public final int offset;

    /** Memory occupied by this field */

    public final int length;

    /** Offset of the first cell in the array */

    public final int arrayBase;

    /** Size of a cell in the array */

    public final int arrayElementSize;

    /** Memory occupied by underlying array (shallow), if this is array type */

    public final int arraySize;

    /** This object fields */

    public final List<ObjectInfo> children;  

    public ObjectInfo(String name, String type, String contents, int offset, int length, int arraySize,

    int arrayBase, int arrayElementSize)

    {

        this.name = name;

        this.type = type;

        this.contents = contents;

        this.offset = offset;

        this.length = length;

        this.arraySize = arraySize;

        this.arrayBase = arrayBase;

        this.arrayElementSize = arrayElementSize;

        children = new ArrayList<ObjectInfo>( 1 );

    }  

    public void addChild( final ObjectInfo info )

    {

        if ( info != null )

            children.add( info );

    }  

    /**

    * Get the full amount of memory occupied by a given object. This value may be slightly less than

    * an actual value because we don't worry about memory alignment - possible padding after the last object field.

    *

    * The result is equal to the last field offset + last field length + all array sizes + all child objects deep sizes

    * @return Deep object size

    */

    public long getDeepSize()

    {

        //return length + arraySize + getUnderlyingSize( arraySize != 0 );

        return addPaddingSize(arraySize + getUnderlyingSize( arraySize != 0 ));

    }  

    long size = 0;  

    private long getUnderlyingSize( final boolean isArray )

    {

        //long size = 0;

        for ( final ObjectInfo child : children )

            size += child.arraySize + child.getUnderlyingSize( child.arraySize != 0 );

        if ( !isArray && !children.isEmpty() ){

            int tempSize = children.get( children.size() - 1 ).offset + children.get( children.size() - 1 ).length;

            size += addPaddingSize(tempSize);

        }  

        return size;

    }  

    private static final class OffsetComparator implements Comparator<ObjectInfo>

    {

        @Override

        public int compare( final ObjectInfo o1, final ObjectInfo o2 )

        {

            return o1.offset - o2.offset; //safe because offsets are small non-negative numbers

        }

    }  

    //sort all children by their offset

    public void sort()

    {

        Collections.sort( children, new OffsetComparator() );

    }  

    @Override

    public String toString() {

        final StringBuilder sb = new StringBuilder();

        toStringHelper( sb, 0 );

        return sb.toString();

    }  

    private void toStringHelper( final StringBuilder sb, final int depth )

    {

        depth( sb, depth ).append("name=").append( name ).append(", type=").append( type )

            .append( ", contents=").append( contents ).append(", offset=").append( offset )

            .append(", length=").append( length );

        if ( arraySize > 0 )

        {

            sb.append(", arrayBase=").append( arrayBase );

            sb.append(", arrayElemSize=").append( arrayElementSize );

            sb.append( ", arraySize=").append( arraySize );

        }

        for ( final ObjectInfo child : children )

        {

            sb.append( '\n' );

            child.toStringHelper(sb, depth + 1);

        }

    }  

    private StringBuilder depth( final StringBuilder sb, final int depth )

    {

        for ( int i = 0; i < depth; ++i )

            sb.append( "\t");

        return sb;

    }  

    private long addPaddingSize(long size){

        if(size % 8 != 0){

            return (size / 8 + 1) * 8;

        }

        return size;

    }  

}  

package test;  

import java.lang.reflect.Array;

import java.lang.reflect.Field;

import java.lang.reflect.Modifier;

import java.util.ArrayList;

import java.util.Arrays;

import java.util.Collections;

import java.util.HashMap;

import java.util.IdentityHashMap;

import java.util.List;

import java.util.Map;  

import sun.misc.Unsafe;  

/**

 * This class could be used for any object contents/memory layout printing.

 */

public class ClassIntrospector {  

    private static final Unsafe unsafe;

    /** Size of any Object reference */

    private static final int objectRefSize;

    static {

        try {

            Field field = Unsafe.class.getDeclaredField("theUnsafe");

            field.setAccessible(true);

            unsafe = (Unsafe) field.get(null);  

            objectRefSize = unsafe.arrayIndexScale(Object[].class);

        } catch (Exception e) {

            throw new RuntimeException(e);

        }

    }  

    /** Sizes of all primitive values */

    private static final Map<Class, Integer> primitiveSizes;  

    static {

        primitiveSizes = new HashMap<Class, Integer>(10);

        primitiveSizes.put(byte.class, 1);

        primitiveSizes.put(char.class, 2);

        primitiveSizes.put(int.class, 4);

        primitiveSizes.put(long.class, 8);

        primitiveSizes.put(float.class, 4);

        primitiveSizes.put(double.class, 8);

        primitiveSizes.put(boolean.class, 1);

    }  

    /**

     * Get object information for any Java object. Do not pass primitives to

     * this method because they will boxed and the information you will get will

     * be related to a boxed version of your value.

     *

     * @param obj

     *            Object to introspect

     * @return Object info

     * @throws IllegalAccessException

     */

    public ObjectInfo introspect(final Object obj)

            throws IllegalAccessException {

        try {

            return introspect(obj, null);

        } finally { // clean visited cache before returning in order to make

                    // this object reusable

            m_visited.clear();

        }

    }  

    // we need to keep track of already visited objects in order to support

    // cycles in the object graphs

    private IdentityHashMap<Object, Boolean> m_visited = new IdentityHashMap<Object, Boolean>(

            100);  

    private ObjectInfo introspect(final Object obj, final Field fld)

            throws IllegalAccessException {

        // use Field type only if the field contains null. In this case we will

        // at least know what's expected to be

        // stored in this field. Otherwise, if a field has interface type, we

        // won't see what's really stored in it.

        // Besides, we should be careful about primitives, because they are

        // passed as boxed values in this method

        // (first arg is object) - for them we should still rely on the field

        // type.

        boolean isPrimitive = fld != null && fld.getType().isPrimitive();

        boolean isRecursive = false; // will be set to true if we have already

                                        // seen this object

        if (!isPrimitive) {

            if (m_visited.containsKey(obj))

                isRecursive = true;

            m_visited.put(obj, true);

        }  

        final Class type = (fld == null || (obj != null && !isPrimitive)) ? obj

                .getClass() : fld.getType();

        int arraySize = 0;

        int baseOffset = 0;

        int indexScale = 0;

        if (type.isArray() && obj != null) {

            baseOffset = unsafe.arrayBaseOffset(type);

            indexScale = unsafe.arrayIndexScale(type);

            arraySize = baseOffset + indexScale * Array.getLength(obj);

        }  

        final ObjectInfo root;

        if (fld == null) {

            root = new ObjectInfo("", type.getCanonicalName(), getContents(obj,

                    type), 0, getShallowSize(type), arraySize, baseOffset,

                    indexScale);

        } else {

            final int offset = (int) unsafe.objectFieldOffset(fld);

            root = new ObjectInfo(fld.getName(), type.getCanonicalName(),

                    getContents(obj, type), offset, getShallowSize(type),

                    arraySize, baseOffset, indexScale);

        }  

        if (!isRecursive && obj != null) {

            if (isObjectArray(type)) {

                // introspect object arrays

                final Object[] ar = (Object[]) obj;

                for (final Object item : ar)

                    if (item != null)

                        root.addChild(introspect(item, null));

            } else {

                for (final Field field : getAllFields(type)) {

                    if ((field.getModifiers() & Modifier.STATIC) != 0) {

                        continue;

                    }

                    field.setAccessible(true);

                    root.addChild(introspect(field.get(obj), field));

                }

            }

        }  

        root.sort(); // sort by offset

        return root;

    }  

    // get all fields for this class, including all superclasses fields

    private static List<Field> getAllFields(final Class type) {

        if (type.isPrimitive())

            return Collections.emptyList();

        Class cur = type;

        final List<Field> res = new ArrayList<Field>(10);

        while (true) {

            Collections.addAll(res, cur.getDeclaredFields());

            if (cur == Object.class)

                break;

            cur = cur.getSuperclass();

        }

        return res;

    }  

    // check if it is an array of objects. I suspect there must be a more

    // API-friendly way to make this check.

    private static boolean isObjectArray(final Class type) {

        if (!type.isArray())

            return false;

        if (type == byte[].class || type == boolean[].class

                || type == char[].class || type == short[].class

                || type == int[].class || type == long[].class

                || type == float[].class || type == double[].class)

            return false;

        return true;

    }  

    // advanced toString logic

    private static String getContents(final Object val, final Class type) {

        if (val == null)

            return "null";

        if (type.isArray()) {

            if (type == byte[].class)

                return Arrays.toString((byte[]) val);

            else if (type == boolean[].class)

                return Arrays.toString((boolean[]) val);

            else if (type == char[].class)

                return Arrays.toString((char[]) val);

            else if (type == short[].class)

                return Arrays.toString((short[]) val);

            else if (type == int[].class)

                return Arrays.toString((int[]) val);

            else if (type == long[].class)

                return Arrays.toString((long[]) val);

            else if (type == float[].class)

                return Arrays.toString((float[]) val);

            else if (type == double[].class)

                return Arrays.toString((double[]) val);

            else

                return Arrays.toString((Object[]) val);

        }

        return val.toString();

    }  

    // obtain a shallow size of a field of given class (primitive or object

    // reference size)

    private static int getShallowSize(final Class type) {

        if (type.isPrimitive()) {

            final Integer res = primitiveSizes.get(type);

            return res != null ? res : 0;

        } else

            return objectRefSize;

    }

}

先一个测试类来验证一下Unsafe的方式计算出的结果

public class ClassIntrospectorTest

{

    public static void main(String[] args) throws IllegalAccessException {

        final ClassIntrospector ci = new ClassIntrospector();  

        ObjectInfo res;  

        res = ci.introspect( new ObjectA() );

        System.out.println( res.getDeepSize() );

    }  

    private static class ObjectA {

        String str;  // 4

        int i1; // 4

        byte b1; // 1

        byte b2; // 1

        int i2;  // 4

        ObjectB obj; //4

        byte b3;  // 1

    }  

    private static class ObjectB {  

    }

}

计算结果如下： 32

和我们之前计算结果是一致的，证明是正确的。

最后再来测试一下数组对象的长度。有两个类如下:

private static class ObjectC {

        ObjectD[] array = new ObjectD[2];

    }  

    private static class ObjectD {

        int value;

    }

它们在内存的大体分布如下图:

我们可以手工计算一下ObjectC obj = new ObjectC()的大小：

ObjectC的Shallow size = 8(_mark) + 4(oop指针) + 4(ObjectD[]引用) = 16

new ObjectD[2]数组的长度 = 8(_mark) + 4(oop指针) + 4(数组长度占4个字节) + 4(ObjectD[0]引用) + 4(ObjectD[1]引用) = 24

由于ObjectD[]数组没有指向具体的对象大小，所以我们手工计算的结果是16 + 24 = 40

使用Unsafe对象的方式来计算一下:

public static void main(String[] args) throws IllegalAccessException {

        final ClassIntrospector ci = new ClassIntrospector();  

        ObjectInfo res;  

        res = ci.introspect( new ObjectC() );

        System.out.println( res.getDeepSize() );

    }

计算结果如下，和我们计算的结果是一致的，证明是正确的: 40

再给ObjectD[]数组指向具体的ObjectD对象，再测试一下结果：

public static void main(String[] args) throws IllegalAccessException {

       final ClassIntrospector ci = new ClassIntrospector();  

       ObjectInfo res;  

       res = ci.introspect( new ObjectC() );

       System.out.println( res.getDeepSize() );

   }  

   private static class ObjectC {

    ObjectD[] array = new ObjectD[2];  

    public ObjectC(){

        array[0] = new ObjectD();

        array[1] = new ObjectD();

    }

   }  

   private static class ObjectD {

    int value;

   }

我们可以手工计算一下ObjectC obj = new ObjectC()的大小： ObjectC的Shallow size = 8(_mark) + 4(oop指针) + 4(ObjectD[]引用) = 16

new ObjectD[2]数组的长度 = 8(_mark) + 4(oop指针) + 4(数组长度占4个字节) + 4(ObjectD[0]引用) + 4(ObjectD[1]引用) = 24

ObjectD对象长度 = 8(_mark) + 4(oop指针) + 4(value) = 16

所以ObjectC实际占用的空间 = 16 + 24 + 2 * 16 = 72

使用Unsafe的方式计算的结果也是72，和我们手工计算的方式一致。

码农公寓

1 基础知识

2 instrument的getObjectSize(obj)

3 sun.misc.Unsafe的方式

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