Java 10：用于增强for循环的字节代码生成[重复]

Question

List data = new ArrayList<>();for (String b : data); 

public class Test

{
  public Test() {}
  public static void main(String[] paramArrayOfString) throws IOException {
    ArrayList localArrayList = new ArrayList();
    String str;
    for (Iterator localIterator = localArrayList.iterator(); localIterator.hasNext(); str = (String)localIterator.next()) {}
}

{
    Iterator iterator = data.iterator();
    for (; iterator.hasNext();) 
    {
        String b = (String)iterator.next();
    }
    b = null;
    iterator = null;
}

Answer 1

编辑：已经存在一个相关的问题：Java“for”语句实现阻止了垃圾收集，而垃圾收集提供了更多信息。

在阅读了bug报告(https://bugs.openjdk.java.net/browse/jdk-8192858和https://bugs.openjdk.java.net/browse/jdk-8175883)之后，可以总结出以下原因：

javac生成的字节码中存在一个问题，导致在循环完成后保留对数组/迭代的引用。

这里的这个用例（摘自bug报告）演示了这一点：

public class IteratorInOneScope { 

    private static final int HALF_OF_MEMORY = (int) (Runtime.getRuntime().maxMemory() * 0.5); 

    public static void main(String[] args) { 
        byte[] data = new byte[HALF_OF_MEMORY]; 

        for (byte b : data); // <-- if you comment this line - the application finished successfully 
        data = null; // this expects to discard reference -> allow to release the memory 

        byte[] data2 = new byte[HALF_OF_MEMORY]; // the memory can't be allocated second time, if the "for" loop statement above is used 

        System.out.println("Success"); 
    } 
}

它编译为以下字节码：

 0: getstatic #2 // Field HALF_OF_MEMORY:I 
 3: newarray byte 
 5: astore_0 <==== array ref in slot #0 

 6: aload_0 
 7: astore_1 <==== array ref in slot #1 

 8: aload_1 
 9: arraylength 
10: istore_2 

11: iconst_0 
12: istore_3 

13: iload_3 
14: iload_2 
15: if_icmpge 29 

18: aload_1 
19: iload_3 
20: baload 
21: istore 4 
23: iinc 3, 1 
26: goto 13 

29: aconst_null 
30: astore_0 <== nulls slot #0 

31: getstatic #2 // Field HALF_OF_MEMORY:I 
34: newarray byte 
36: astore_1 
37: getstatic #3 // Field java/lang/System.out:Ljava/io/PrintStream; 
40: ldc #4 // String Success 
42: invokevirtual #5 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 
45: return 

JLS 14.14.2没有更新，原因如下：