无用知识之获取对象内存占用大小

无脑记录一下

首先是这个工具类

public class ObjectSizeFetcher {
    private static Instrumentation instrumentation;

    public static void premain(String args, Instrumentation inst) {
        instrumentation = inst;
    }

    public static long getObjectSize(Object o) {
        return instrumentation.getObjectSize(o);
    }

    public static void main(String[] args) {
        String filename = args.length > 0 ? args[0] : "pinyin.properties";
        Properties pairs = PinyinResource.getResource(filename);
        long size = getObjectSize(pairs);
        long total = 0;
        long entryTotal = 0;
        for (Map.Entry<Object, Object> entry : pairs.entrySet()) {
            Object key = entry.getKey();
            Object value = entry.getValue();
            total += getObjectSize(key);
            total += getObjectSize(value);
            entryTotal += getObjectSize(entry);
        }

        System.out.println("used bytes of props " + size + ", of entries " + entryTotal
                + ", of object values " + total);

        for (int i = 1; i < args.length; i++) {
            System.out.println(args[i] + " takes " + getObjectSize(args[i]) + " bytes, chars tooks " + getObjectSize(args[i].toCharArray()));
        }

    }
}

其次是编译成class文件 -> 用idea的运行
或者

javac dir/*.java -d dir

创建一个声明了premain和main类的manifest.mf文件

Manifest-Version: 1.0
Main-Class: com.ssynhtn.text.ObjectSizeFetcher
Premain-Class: com.ssynhtn.text.ObjectSizeFetcher

打包

jar cmf dir/manifest.mf hello.jar dir/*.class

指定agent运行, 至于agent是啥先不管了, 反正是和premain类组合, 会先调用premian方法的一个东西

java -javaagent:hello.jar -jar hello.jar args

运行结果

used bytes of props 48, of entries 668896, of object values 1003344
hello takes 24 bytes, chars tooks 32
worldworl takes 24 bytes, chars tooks 40
我 takes 24 bytes, chars tooks 24

2万个key->value组成的Properties类, 如果只计算它的entry和string, 至少占用了1.6MB的内存

似乎好像有点对不上呢, 不过先不管了, 总之String的内存占用比想象的要高的样子