Java8 metaspace&堆使用情况

问题描述

我有这个代码动态生成类并加载它

I have this code to generate class dynamically and load it

import javassist.CannotCompileException;
import javassist.ClassPool;

public class PermGenLeak {
    private static final String PACKAGE_NAME = "com.jigarjoshi.permgenleak.";

    public static void main(String[] args) throws CannotCompileException, InterruptedException {
        for (int i = 0; i < Integer.MAX_VALUE; i++) {
            ClassPool pool = ClassPool.getDefault();
            pool.makeClass(PACKAGE_NAME + i).toClass();
            Thread.sleep(3);
        }

    }
}

我推出这个类对Java 7（jdk1.7.0_60）和预期它填满了PermGenSpace并且堆仍未使用

I launched this class against Java 7 (jdk1.7.0_60) and as expected it filled up PermGenSpace and heap remained unused

现在相同的代码针对Java 8运行（jdk1 .8.0_40-ea）并且正如预期的那样它不断扩大本机内存（Metaspace）但令人惊讶的是，1g Metaspace在OldGen中消耗了3g Heap（随着时间的推移几乎是Metaspace的3倍）

Now the same code ran against Java 8 (jdk1.8.0_40-ea) and as expected it kept expanding native memory (Metaspace) but surprisingly for 1g of Metaspace it consumed 3g of Heap in OldGen (almost 3x of Metaspace maintained over time)

和说

在将多个类加载到Metaspace中时，堆的增加究竟是什么？

what exactly makes this increase in heap as it loads more classes into Metaspace ?

推荐答案

运行 jmap -histo PID 以查看哪些对象占用堆空间。

当我运行你的例子时，我看到堆满了Javassist辅助对象：

Run jmap -histo PID to see which objects consume the Heap Space.
When I ran your example I saw the heap full of Javassist auxiliary objects:

 num     #instances         #bytes  class name
----------------------------------------------
   1:        592309      312739152  [Ljavassist.bytecode.ConstInfo;
   2:       6515673      208501536  java.util.HashMap$Node
   3:       2964403      169188824  [C
   4:       1777622      102165184  [Ljava.lang.Object;
   5:       4146200       99508800  javassist.bytecode.Utf8Info
   6:       3553889       85293336  java.util.ArrayList
   7:       2964371       71144904  java.lang.String
   8:        593075       56944008  java.lang.Class
   9:        592332       47388032  [Ljava.util.HashMap$Node;
  10:        592309       37907776  javassist.bytecode.ClassFile
  11:        592308       37907712  javassist.CtNewClass
  12:       1185118       28555808  [B
  13:        592342       28432416  java.util.HashMap
  14:       1184624       28430976  javassist.bytecode.ClassInfo
  15:        592309       28430832  [[Ljavassist.bytecode.ConstInfo;
  16:        592322       23692880  javassist.bytecode.MethodInfo
  17:        592315       23692600  javassist.bytecode.CodeAttribute
  18:        592434       18957888  java.util.Hashtable$Entry
  19:        592309       18953888  javassist.bytecode.ConstPool
  20:        592308       18953856  java.lang.ref.WeakReference
  21:        592318       14215632  javassist.bytecode.MethodrefInfo
  22:        592318       14215632  javassist.bytecode.NameAndTypeInfo
  23:        592315       14215560  javassist.bytecode.ExceptionTable
  24:        592309       14215416  javassist.bytecode.LongVector
  25:        592309       14215416  javassist.bytecode.SourceFileAttribute
  26:        592507        9487584  [I
  27:             8        6292528  [Ljava.util.Hashtable$Entry;
  28:           212          18656  java.lang.reflect.Method
  29:           407          13024  java.util.concurrent.ConcurrentHashMap$Node
  30:           124           8928  java.lang.reflect.Field

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