在jdk1.8的環境下模擬永久代內存溢出

相信很多小夥伴在看深刻理解Java虛擬機的時候，做者給咱們舉例一個demo來發生PermGen space

一、經過List不斷添加String.intern();

二、經過設置對應的-XX:PermSize與-XX:MaxPermSize(更快看到效果)，

三、在jdk1.6的環境下會拋出OOM:PermGen space異常

public static void main(String[] args) {
    List<String> str=new ArrayList<>();
    int i=0;
    while (true){
        str.add(String.valueOf(i).intern());
    }

}

然而在jdk1.8的環境下，這段代碼，會出現OOM，但不是出現PermGenSpace，而是會當堆內存不夠用(-Xmx)的時候，拋出Java heap space，並且會舒適提示

ignoring option PermSize=10M; support was removed in 8.0

ignoring option MaxPermSize=10M; support was removed in 8.0

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首先咱們看一下String.intern()方法，官方的註釋是

Returns a canonical representation for the string object. A pool of strings, initially empty, is maintained privately by the class String. When the intern method is invoked, if the pool already contains a string equal to this String object as determined by the equals(Object) method, then the string from the pool is returned. Otherwise, this String object is added to the pool and a reference to this String object is returned. It follows that for any two strings s and t, s.intern() == t.intern() is true if and only if s.equals(t) is true. All literal strings and string-valued constant expressions are interned. String literals are defined in section 3.10.5 of the The Java? Language Specification.

大概意思就是，若是常量池已經存在這個string對象，那麼就返回這個字符串的引用。不然將此string對象加入常量池，再返回引用

~

可是在jdk1.8中，其實已經沒有永久代這一說了，取而代之的是一個叫元空間(Meta space)。而常量池放到了堆中，因此也就不會出現PermGen space了

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那麼若是想看到metaspace的異常怎麼作呢？

一個是能夠把這兩個值設置的足夠小，那麼啓動就會報錯了。

-XX:MetaspaceSize=3M -XX:MaxMetaspaceSize=3M

一個是能夠使用jdk動態加載技術，例如cglib動態的生成大量的數據來達到