Java“for”语句实现防止垃圾收集

Question

UPD 21.11.2017：该bug在JDK中被修复，参见Vicente Romero的评论

摘要：

如果for语句用于任何iterable实现，则集合将保留在堆内存中，直到当前作用域（方法、语句体）结束，即使您没有对集合的任何其他引用，并且应用程序需要分配新内存，也不会被垃圾收集。

import java.util.ArrayList;
public class IteratorAndGc {
    
    // number of strings and the size of every string
    static final int N = 7500;

    public static void main(String[] args) {
        System.gc();

        gcInMethod();

        System.gc();
        showMemoryUsage("GC after the method body");

        ArrayList<String> strings2 = generateLargeStringsArray(N);
        showMemoryUsage("Third allocation outside the method is always successful");
    }

    // main testable method
    public static void gcInMethod() {

        showMemoryUsage("Before first memory allocating");
        ArrayList<String> strings = generateLargeStringsArray(N);
        showMemoryUsage("After first memory allocation");


        // this is only one difference - after the iterator created, memory won't be collected till end of this function
        for (String string : strings);
        showMemoryUsage("After iteration");

        strings = null; // discard the reference to the array

        // one says this doesn't guarantee garbage collection,
        // Oracle says "the Java Virtual Machine has made a best effort to reclaim space from all discarded objects".
        // but no matter - the program behavior remains the same with or without this line. You may skip it and test.
        System.gc();

        showMemoryUsage("After force GC in the method body");

        try {
            System.out.println("Try to allocate memory in the method body again:");
            ArrayList<String> strings2 = generateLargeStringsArray(N);
            showMemoryUsage("After secondary memory allocation");
        } catch (OutOfMemoryError e) {
            showMemoryUsage("!!!! Out of memory error !!!!");
            System.out.println();
        }
    }
    
    // function to allocate and return a reference to a lot of memory
    private static ArrayList<String> generateLargeStringsArray(int N) {
        ArrayList<String> strings = new ArrayList<>(N);
        for (int i = 0; i < N; i++) {
            StringBuilder sb = new StringBuilder(N);
            for (int j = 0; j < N; j++) {
                sb.append((char)Math.round(Math.random() * 0xFFFF));
            }
            strings.add(sb.toString());
        }

        return strings;
    }

    // helper method to display current memory status
    public static void showMemoryUsage(String action) {
        long free = Runtime.getRuntime().freeMemory();
        long total = Runtime.getRuntime().totalMemory();
        long max = Runtime.getRuntime().maxMemory();
        long used = total - free;
        System.out.printf("\t%40s: %10dk of max %10dk%n", action, used / 1024, max / 1024);
    }
}

javac IteratorAndGc.java   &&   java -Xms180m -Xmx180m IteratorAndGc

在运行时，我有：

第一次内存分配前：最大176640K的1251K

第一次内存分配后：最大176640K的131426K

     !!!! Out of memory error !!!!:     168948k of max     176640k

为了避免在函数体中出现这种OutOfMemoryError，只需删除/注释这一行即可：

用于（String String：strings）；<--这就是邪恶！！！

然后输出如下所示：

因此，在没有for迭代的情况下，在丢弃对字符串的引用后成功收集垃圾，并在第二次（在函数体内）和第三次（在方法外）分配垃圾。

我的假设：

语法构造编译为

Iterator iter = strings.iterator();
while(iter.hasNext()){
    iter.next()
}

Iterator iter = strings.iterator();
while(iter.hasNext()){
    iter.next()
}
iter = null; // <--- flush the water!

javac 1.8.0_111

java version "1.8.0_111"
Java(TM) SE Runtime Environment (build 1.8.0_111-b14)
Java HotSpot(TM) 64-Bit Server VM (build 25.111-b14, mixed mode)

问题不是关于system.gc()行为（您可以从示例中删除所有gc调用）--在第二个字符串分配期间，JVM必须释放dicarded内存。

引用测试java类，在线编译器进行测试（但是这个资源只有50 Mb的堆，所以使用N=5000)

Answer 1

最后，接受、批准并修复Oracle/Open JKD bug：

http://bugs.java.com/bugdatabase/view_bug.do？bug_id=jdk-8175883

https://bugs.openjdk.java.net/browse/jdk-8175883