JVM笔记三：Java内存区域与内存溢出异常——OutOfMemoryError异常

文章目录
  1. 1. Java堆溢出
    1. 1.1. 虚拟机栈和本地方法栈溢出
    2. 1.2. 方法区和运行时常量池溢出

Java堆溢出

Java堆用于存储对象实例，只要不断地创建对象，并且保证GC Roots到对象之间有可达路径来避免垃圾回收机制清除这些对象，那么在对象数量到达最大的容量限制后就会产生内存溢出异常。

下面代码限制Java堆的大小为20MB，不可扩展（将堆的最小值-Xms参数与最大值-Xmx参数设置为一样即可避免堆自动扩展），通过参数-XX:+HeapDumpOnOutOfMemoryError可以让虚拟机在出现内存溢出时Dump出当前的内存堆转储快照以便事后进行分析。

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/**
 * HeapOOM
 * Created by larry.su on 2017/2/7.
 * -Xms20m -Xmx20m -XX:+HeapDumpOnOutOfMemoryError
 */
public class HeapOOM {
    static class OOMObject {}

    public static void main(String[] args) {
        List<OOMObject> list = new ArrayList<OOMObject>();
        while (true) {
            list.add(new OOMObject());
        }
    }
}

运行结果：

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java.lang.OutOfMemoryError: Java heap space
Dumping heap to java_pid65766.hprof ...
Heap dump file created [27784157 bytes in 0.137 secs]
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space

虚拟机栈和本地方法栈溢出

HotSpot虚拟机中并不区分虚拟机栈和本地方法栈，对于HotSpot来说，虽然-Xoss参数（设置本地方法栈大小）存在，但实际上是无效的。栈容量只由-Xss参数设定。

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/**
 * VM Args: -Xss160k
 * Created by larry.su on 2017/2/17.
 */
public class JavaVMStackSOF {
    private int stackLength = 1;

    public void stackLeak(){
        stackLength ++ ;
        stackLeak();
    }

    public static void main(String[] args) throws Throwable {
        JavaVMStackSOF oom = new JavaVMStackSOF();
        try {
            oom.stackLeak();
        } catch (Throwable e) {
            System.out.println("stackLength:" + oom.stackLength);
            throw  e;
        }
    }
}

运行结果：

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Exception in thread "main" java.lang.StackOverflowError
stackLength:744
    at com.jvm.JavaVMStackSOF.stackLeak(JavaVMStackSOF.java:14)
    at com.jvm.JavaVMStackSOF.stackLeak(JavaVMStackSOF.java:15)
    at com.jvm.JavaVMStackSOF.stackLeak(JavaVMStackSOF.java:15)

在单线程下，无论是由于栈帧太大还是虚拟机栈帧容量太小，当内存无法分配的时候，虚拟机抛出的都是StackOverFlowError异常。

方法区和运行时常量池溢出

由于运行时常量池是方法区的一部分，因此这两个区域的溢出测试就放在一起进行。方法区用于存放Class的相关信息，如类名、访问修饰符、常量池、字段描述、方法描述等。

String.intern()。在JDK1.6之前版本中由于常量池分配在永久内存中，我们可以通过-XX:PermSize和-XX:MaxPermSize限制方法区的大小，从而间接限制其中常量池的容量。

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/**
 * Returns a canonical representation for the string object.
 * <p>
 * A pool of strings, initially empty, is maintained privately by the
 * class {@code String}.
 * <p>
 * When the intern method is invoked, if the pool already contains a
 * string equal to this {@code String} object as determined by
 * the {@link #equals(Object)} method, then the string from the pool is
 * returned. Otherwise, this {@code String} object is added to the
 * pool and a reference to this {@code String} object is returned.
 * <p>
 * It follows that for any two strings {@code s} and {@code t},
 * {@code s.intern() == t.intern()} is {@code true}
 * if and only if {@code s.equals(t)} is {@code true}.
 * <p>
 * All literal strings and string-valued constant expressions are
 * interned. String literals are defined in section 3.10.5 of the
 * <cite>The Java&trade; Language Specification</cite>.
 *
 * @return  a string that has the same contents as this string, but is
 *          guaranteed to be from a pool of unique strings.
 */
public native String intern();
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/**
 * VM Args: -XX:PermSize=10M -XX:MaxPermSize=10M
 * Created by larry.su on 2017/2/17.
 */
public class RuntimeConstantsPoolOOM {
    public static void main(String[] args) throws Throwable {
        //使用List保持常量池的引用，避免Full GC回收常量池行为
        List<String> list = new ArrayList<String>();
        //10M 的PermSize在Integer范围内足够产生OOM
        int i = 0;
        while (true) {
            try {
                list.add(String.valueOf(i++).intern());
            } catch (Throwable e) {
                System.out.println(i);
                throw e;
            }
        }
    }
}

运行结果：

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86889
Exception in thread "main" java.lang.OutOfMemoryError: PermGen space
at java.lang.String.intern(Native Method)
at com.jvm.RuntimeConstantsPoolOOM.main(RuntimeConstantsPoolOOM.java:21)

在JDK1.7中运行这段代码就不会得到相同的接口，while会一直循环下去。

在JDK8中，while同样一直循环下去，同时控制台打印如下信息：

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Java HotSpot(TM) 64-Bit Server VM warning: ignoring option PermSize=10M; support was removed in 8.0
Java HotSpot(TM) 64-Bit Server VM warning: ignoring option MaxPermSize=10M; support was removed in 8.0