如何查看java对象所占的内存大小

发布网友发布时间：2022-05-06 06:51

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热心网友时间：2022-06-28 23:00

我们为什么想要知道对象所占对内存的大小呢？（1）做一些cache的时候，我们不可能把数据库的所有的数据都缓存到内存里面，我们要估计缓存的大小。（2）内存泄露的时候，我们可以查看某些对象的大小来定位问题，当然还有其他的更有效的方式，比如使用MAT分析mp文件（3）根据jvm的堆内存设置，我们可以知道最多可以创建多少个对象。从jdk5开始，提供了Instrumentation API，它有一个叫做getObjectSize()的方法，但是，这个方法存在两个问题：（1）不可以直接使用。必须要实现一个Instrumentation Agent，还得放到jar包里面。（2）它只能返回单个对象的大小，不能返回内部包含的子对象的大小。完整的代码如下： package com.bj58.test; import java.lang.instrument.Instrumentation; import java.lang.reflect.Array; import java.lang.reflect.Field; import java.lang.reflect.Modifier; import java.util.IdentityHashMap; import java.util.Map; import java.util.Stack; public class SizeOfAgent { static Instrumentation inst; /** initializes agent */ public static void premain(String agentArgs, Instrumentation instP) { inst = instP; } /** * Returns object size without member sub-objects. * * @param o * object to get size of * @return object size */ public static long sizeOf(Object o) { if (inst == null) { throw new IllegalStateException( "Can not access instrumentation environment.\n" + "Please check if jar file containing SizeOfAgent class is \n" + "specified in the java's \"-javaagent\" command line argument."); } return inst.getObjectSize(o); } /** * Calculates full size of object iterating over its hierarchy graph. * * @param obj * object to calculate size of * @return object size */ public static long fullSizeOf(Object obj) { Map<Object, Object> visited = new IdentityHashMap<Object, Object>(); Stack<Object> stack = new Stack<Object>(); long result = internalSizeOf(obj, stack, visited); while (!stack.isEmpty()) { result += internalSizeOf(stack.pop(), stack, visited); } visited.clear(); return result; } private static boolean skipObject(Object obj, Map<Object, Object> visited) { if (obj instanceof String) { // skip interned string if (obj == ((String) obj).intern()) { return true; } } return (obj == null) // skip visited object || visited.containsKey(obj); } private static long internalSizeOf(Object obj, Stack<Object> stack, Map<Object, Object> visited) { if (skipObject(obj, visited)) { return 0; } visited.put(obj, null); long result = 0; // get size of object + primitive variables + member pointers result += SizeOfAgent.sizeOf(obj); // process all array elements Class clazz = obj.getClass(); if (clazz.isArray()) { if (clazz.getName().length() != 2) {// skip primitive type array int length = Array.getLength(obj); for (int i = 0; i < length; i++) { stack.add(Array.get(obj, i)); } } return result; } // process all fields of the object while (clazz != null) { Field[] fields = clazz.getDeclaredFields(); for (int i = 0; i < fields.length; i++) { if (!Modifier.isStatic(fields[i].getModifiers())) { if (fields[i].getType().isPrimitive()) { continue; // skip primitive fields } else { fields[i].setAccessible(true); try { // objects to be estimated are put to stack Object objectToAdd = fields[i].get(obj); if (objectToAdd != null) { stack.add(objectToAdd); } } catch (IllegalAccessException ex) { assert false; } } } } clazz = clazz.getSuperclass(); } return result; } } 然后我们可以做一个测试： public class Test { static class Person{ private int id; private String name; private String address; public Person(int id, String name, String address) { this.id = id; this.name = name; this.address = address; } } public static void main(String[] args) throws Exception { Person p = new Person(12, "xujsh","bj"); long size = SizeOfAgent.fullSizeOf(p); System.out.println(size); } } 切换到命令行： D:\workspace\objsize\src>java -version java version "1.6.0_22" Java(TM) SE Runtime Environment (build 1.6.0_22-b04) Java HotSpot(TM) Client VM (build 17.1-b03, mixed mode, sharing) D:\workspace\objsize\src>javac com/bj58/test/*.java D:\workspace\objsize\src>jar -cvfm size.jar MANIFEST.MF com/bj58/test/* 标明清单(manifest) 增加：com/bj58/test/SizeOfAgent.class(读入= 3119) (写出= 1698)(压缩了 45%) 增加：com/bj58/test/SizeOfAgent.java(读入= 3147) (写出= 1204)(压缩了 61%) 增加：com/bj58/test/Test$Person.class(读入= 442) (写出= 305)(压缩了 30%) 增加：com/bj58/test/Test.class(读入= 692) (写出= 441)(压缩了 36%) 增加：com/bj58/test/Test.java(读入= 509) (写出= 290)(压缩了 43%) D:\workspace\objsize\src>java -javaagent:size.jar com.bj58.test.Test 24 MANIFEST.MF： Manifest-Version: 1.0 Main-Class: com.bj58.test.Test Premain-Class: com.bj58.test.SizeOfAgent Boot-Class-Path: Can-Redefine-Classes: false 【注意】MANIFEST.MF文件的格式要求比较严格，每一行要满足：key:空格value回车如何在web应用程序里面使用呢？