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JVM系列 - 内存异常的代码实践

Oct 299, 29029
Oct 289, 28089

Java 堆溢出

条件

  1. 不断地创建对象
  2. 保证 GC Roots 到对象之间可达，避免垃圾回收
  3. 通过 -Xmx 和 -Xms 设置堆的最大值和最小值

代码与报错

/**
 * VM Args: -Xms10m -Xmx10m -XX:+HeapDumpOnOutOfMemoryError
 */
public class HeapOOM {
    static class OOMObject {
        
    }
    
    public static void main(String[] args) {
        List<OOMObject> list = new ArrayList<>();
        while (true) {
            list.add(new OOMObject());
        }
    }
}

java.lang.OutOfMemoryError: Java heap space Dumping heap to java_pid6584.hprof … Heap dump file created [12984164 bytes in 0.190 secs]

Exception in thread “main” java.lang.OutOfMemoryError: Java heap space

​ at java.util.Arrays.copyOf(Arrays.java:3210) ​ at java.util.Arrays.copyOf(Arrays.java:3181) ​ at java.util.ArrayList.grow(ArrayList.java:261) ​ at java.util.ArrayList.ensureExplicitCapacity(ArrayList.java:235) ​ at java.util.ArrayList.ensureCapacityInternal(ArrayList.java:227) ​ at java.util.ArrayList.add(ArrayList.java:458) ​ at chapter2.HeapOOM.main(HeapOOM.java:21)

解决

内存泄漏

查看泄漏对象到 GC Roots 的引用链，找到垃圾回收器无法回收它们的原因

内存溢出

查看 JVM 的堆参数 -Xmx 和 -Xms 的设置是否合理，对象的生命周期是否不当

JVM 栈 & 本地方法栈溢出

条件

  1. 通过 -Xss 减少栈内存容量
  2. 定义大量局部变量，增大方法 栈帧局部变量表 的大小

代码与报错

/**
 * VM Args: -Xss160k
 */
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("stack length: " + oom.stackLength);
            throw e;
        }
    }
    
}

stack length: 773

Exception in thread “main” java.lang.StackOverflowError

​ at chapter2.JavaVMStackSOF.stackLeak(JavaVMStackSOF.java:13)

​ at chapter2.JavaVMStackSOF.stackLeak(JavaVMStackSOF.java:14)

​ at chapter2.JavaVMStackSOF.stackLeak(JavaVMStackSOF.java:14)

​ at chapter2.JavaVMStackSOF.stackLeak(JavaVMStackSOF.java:14)

​ at chapter2.JavaVMStackSOF.stackLeak(JavaVMStackSOF.java:14)

​ at chapter2.JavaVMStackSOF.stackLeak(JavaVMStackSOF.java:14)

​ at chapter2.JavaVMStackSOF.stackLeak(JavaVMStackSOF.java:14)

​ at chapter2.JavaVMStackSOF.stackLeak(JavaVMStackSOF.java:14)

public class JavaVMStackSOF {

    private static int stackLength = 0;

    public static void test() {
        long unused1, unused2, unused3, unused4, unused5;
        stackLength++;
        test();
        unused1 = unused2 = unused3 = unused4 = unused5 = 0;
    }

    public static void main(String[] args) {
        try {
            test();
        } catch (Error error) {
            System.out.println("stack length: " + stackLength);
            throw error;
        }
    }

}

stack length: 14990

Exception in thread “main” java.lang.StackOverflowError

​ at chapter2.JavaVMStackSOF.test(JavaVMStackSOF.java:15)

​ at chapter2.JavaVMStackSOF.test(JavaVMStackSOF.java:15)

​ at chapter2.JavaVMStackSOF.test(JavaVMStackSOF.java:15)

​ at chapter2.JavaVMStackSOF.test(JavaVMStackSOF.java:15)

​ at chapter2.JavaVMStackSOF.test(JavaVMStackSOF.java:15)

​ at chapter2.JavaVMStackSOF.test(JavaVMStackSOF.java:15)

​ at chapter2.JavaVMStackSOF.test(JavaVMStackSOF.java:15)

​ at chapter2.JavaVMStackSOF.test(JavaVMStackSOF.java:15)

/**
 * VM Args: -Xss2M，在我的 64 位机器上没有复现出 unable to create native thread
 */
public class JavaVMStackOOM {

    private void dontStop() {
        while (true) {

        }
    }

    public void stackLeakByThread() {
        while (true) {
            Thread thread = new Thread(() -> {
                dontStop();
            });
            thread.start();
        }
    }

    public static void main(String[] args) {
        JavaVMStackOOM oom = new JavaVMStackOOM();
        oom.stackLeakByThread();
    }

}

解决

一般从堆栈信息中，容易分析出问题所在

注意：可以适当减少 最大堆 和 栈容量 的方式来换取更多的线程

方法区 & 运行时常量池溢出

条件

  1. 在 JDK 8 之后，运行时常量池放到了 Java 堆中，因此限制 Java 堆大小

代码与报错

/**
 * VM Args: -Xmx6m
 */
public class RuntimeConstantPoolOOM {

    public static void main(String[] args) {
        Set<String> set = new HashSet<>();
        short i = 0;
        while (true) {
            set.add(String.valueOf(i++).intern());
        }
    }
}

Exception in thread “main” java.lang.OutOfMemoryError: Java heap space

​ at java.util.HashMap.resize(HashMap.java:703)

​ at java.util.HashMap.putVal(HashMap.java:662)

​ at java.util.HashMap.put(HashMap.java:611)

​ at java.util.HashSet.add(HashSet.java:219)

/**
 * VM Args: -XX:MetaspaceSize=10M -XX:MaxMetaspaceSize=10M
 */
public class JavaMethodAreaOOM {

    public static void main(String[] args) {
        while (true) {
            Enhancer enhancer = new Enhancer();
            enhancer.setSuperclass(OOMObject.class);
            enhancer.setUseCache(false);
            enhancer.setCallback((MethodInterceptor) (o, method, objects, methodProxy) -> methodProxy.invokeSuper(o, objects));
            enhancer.create();
        }
    }

    static class OOMObject {

    }

}

Exception in thread “main” java.lang.OutOfMemoryError: Metaspace

​ at net.sf.cglib.core.AbstractClassGenerator.create

​ at net.sf.cglib.proxy.Enhancer.createHelper(Enhancer.java:377)

​ at net.sf.cglib.proxy.Enhancer.create(Enhancer.java:285)

​ at chapter2.JavaMethodAreaOOM.main(JavaMethodAreaOOM.java:19)

解决

可以通过 -XX:MaxMetaspaceSize、-XX:MetaspaceSize 等等参数保护元空间

本机直接内存溢出

条件

  1. 通过设置 -XX:MaxDirectMemorySize 较小，达到内存溢出的目的

代码与报错

/**
 * VM Args: -Xmx20M -XX:MaxDirectMemorySize=10M 然而没有复现 OOM
 */
public class DirectMemoryOOM {
    
    private static final int _1MB = 1024 * 1024;
    
    public static void main(String[] args) throws Exception {
        Field unsafeField = Unsafe.class.getDeclaredFields()[0];
        unsafeField.setAccessible(true);
        Unsafe unsafe = (Unsafe) unsafeField.get(null);
        while (true) {
            unsafe.allocateMemory(_1MB);
        }
    }
}

解决

直接内存的大小可以通过 -XX:MaxDirectMemorySize 指定

默认与 Java 堆最大值一致