Java中对象占用内存计算方法

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

1. 通过java.lang.instrument.Instrumentation的getObjectSize(obj)直接获取对象的大小

2. 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

java.lang.instrument.Instrumentation.getObjectSize()的方式

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。

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( '
' );
            child.toStringHelper(sb, depth + 1);
        }
    }

    private StringBuilder depth( final StringBuilder sb, final int depth )
    {
        for ( int i = 0; i < depth; ++i )
            sb.append( "	");
        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。