java for each实现机制

源测试代码如下

public class ForEachTest {
    public void test4Iterate(Iterable<String> strings) {
        for (String str : strings) {
            System.out.println(str);
        }
    }

    public void test4Array(String[] strings) {
        for (String str : strings) {
            System.out.println(str);
        }
    }
}

执行编译命令

javac ForEachTest.java

使用idea查看反编的Java文件

import java.util.Iterator;

public class ForEachTest {
    public ForEachTest() {
    }

    public void test4Iterate(Iterable<String> var1) {
        Iterator var2 = var1.iterator();

        while(var2.hasNext()) {
            String var3 = (String)var2.next();
            System.out.println(var3);
        }

    }

    public void test4Array(String[] var1) {
        String[] var2 = var1;
        int var3 = var1.length;

        for(int var4 = 0; var4 < var3; ++var4) {
            String var5 = var2[var4];
            System.out.println(var5);
        }

    }
}

执行指令查看对应的汇编指令

javap -c ForEachTest.class

Compiled from "ForEachTest.java"
public class ForEachTest {
  public ForEachTest();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public void test4Iterate(java.lang.Iterable<java.lang.String>);
    Code:
       0: aload_1
       1: invokeinterface #2,  1            // InterfaceMethod java/lang/Iterable.iterator:()Ljava/util/Iterator;
       6: astore_2
       7: aload_2
       8: invokeinterface #3,  1            // InterfaceMethod java/util/Iterator.hasNext:()Z
      13: ifeq          36
      16: aload_2
      17: invokeinterface #4,  1            // InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object;
      22: checkcast     #5                  // class java/lang/String
      25: astore_3
      26: getstatic     #6                  // Field java/lang/System.out:Ljava/io/PrintStream;
      29: aload_3
      30: invokevirtual #7                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      33: goto          7
      36: return

  public void test4Array(java.lang.String[]);
    Code:
       0: aload_1
       1: astore_2
       2: aload_2
       3: arraylength
       4: istore_3
       5: iconst_0
       6: istore        4
       8: iload         4
      10: iload_3
      11: if_icmpge     34
      14: aload_2
      15: iload         4
      17: aaload
      18: astore        5
      20: getstatic     #6                  // Field java/lang/System.out:Ljava/io/PrintStream;
      23: aload         5
      25: invokevirtual #7                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      28: iinc          4, 1
      31: goto          8
      34: return
}

由反编类和汇编指令可以看到对于Iterable类，是通过其迭代方法执行for each，而对于数组是通过传统的for循环方式执行迭代

因为集合类的for each是基于迭代器循环的，所以根据迭代器实现的不同,for each的性能也会不同