关于 for 循环外 定义数组长度 length长度 和 直接for循环 内比较数组长度的区别

public class Test2 {

    static long[] images = new long[]{1,2,3,4,5,6,7};

    public static void main(String[] args) {

    }

    private static void format1() throws InterruptedException {
        for (int i = 0; i < images.length; i++) {
            Thread.sleep(images[i]);
        }
    }

    private static void format2() throws InterruptedException {
        int length =images.length;
        for (int i = 0; i < length; i++) {
            Thread.sleep(images[i]);
        }
    }

}

如果不明白java 内存结构的建议先去学习下java 内存结构。

我们的方法在栈中运行 栈内存结构如下：这里需要理解的是 局部变量指的是 基本数据类型
那么 问题是 format2 中的 length 到底是在局部变量中 还是在别的地方 比如常量池 解答如下

首先是通过idea 中 打开字节码文件

展示如下：

/ class version 52.0 (52)
// access flags 0x21
public class com/lagou/example/first/week17/Test2 {

  // compiled from: Test2.java

  // access flags 0x8
  static [J images

  // access flags 0x1
  public ()V
   L0
    LINENUMBER 8 L0
    ALOAD 0
    INVOKESPECIAL java/lang/Object. ()V
    RETURN
   L1
    LOCALVARIABLE this Lcom/lagou/example/first/week17/Test2; L0 L1 0
    MAXSTACK = 1
    MAXLOCALS = 1

  // access flags 0x9
  public static main([Ljava/lang/String;)V
    // parameter  args
   L0
    LINENUMBER 14 L0
    RETURN
   L1
    LOCALVARIABLE args [Ljava/lang/String; L0 L1 0
    MAXSTACK = 0
    MAXLOCALS = 1

  // access flags 0xA
  private static format1()V throws java/lang/InterruptedException 
   L0
    LINENUMBER 17 L0
    ICONST_0
    ISTORE 0
   L1
   frame APPEND [I]
    ILOAD 0
    GETSTATIC com/lagou/example/first/week17/Test2.images : [J
    ARRAYLENGTH
    IF_ICMPGE L2
   L3
    LINENUMBER 18 L3
    GETSTATIC com/lagou/example/first/week17/Test2.images : [J
    ILOAD 0
    LALOAD
    INVOKESTATIC java/lang/Thread.sleep (J)V
   L4
    LINENUMBER 17 L4
    IINC 0 1
    GOTO L1
   L2
    LINENUMBER 20 L2
   frame CHOP 1
    RETURN
    LOCALVARIABLE i I L1 L2 0
    MAXSTACK = 2
    MAXLOCALS = 1

  // access flags 0xA
  private static format2()V throws java/lang/InterruptedException 
   L0
    LINENUMBER 23 L0
    GETSTATIC com/lagou/example/first/week17/Test2.images : [J
    ARRAYLENGTH
    ISTORE 0
   L1
    LINENUMBER 24 L1
    ICONST_0
    ISTORE 1
   L2
   frame APPEND [I I]
    ILOAD 1
    ILOAD 0
    IF_ICMPGE L3
   L4
    LINENUMBER 25 L4
    GETSTATIC com/lagou/example/first/week17/Test2.images : [J
    ILOAD 1
    LALOAD
    INVOKESTATIC java/lang/Thread.sleep (J)V
   L5
    LINENUMBER 24 L5
    IINC 1 1
    GOTO L2
   L3
    LINENUMBER 27 L3
   frame CHOP 1
    RETURN
   L6
    LOCALVARIABLE i I L2 L3 1
    LOCALVARIABLE length I L1 L6 0
    MAXSTACK = 2
    MAXLOCALS = 2

  // access flags 0x8
  static ()V
   L0
    LINENUMBER 10 L0
    BIPUSH 7
    NEWARRAY T_LONG
    DUP
    ICONST_0
    LCONST_1
    LASTORE
    DUP
    ICONST_1
    LDC 2
    LASTORE
    DUP
    ICONST_2
    LDC 3
    LASTORE
    DUP
    ICONST_3
    LDC 4
    LASTORE
    DUP
    ICONST_4
    LDC 5
    LASTORE
    DUP
    ICONST_5
    LDC 6
    LASTORE
    DUP
    BIPUSH 6
    LDC 7
    LASTORE
    PUTSTATIC com/lagou/example/first/week17/Test2.images : [J
    RETURN
    MAXSTACK = 5
    MAXLOCALS = 0
}

可以参考下 字节码 对照表
https://www.cnblogs.com/sheeva/p/6279096.html

可以看到 format1 每次都会去堆中读取 数组的长度信息
而 format2 的int length 数据类型 属于 局部变量
可以对应上面网址中字节码的信息 大家对比下

There is a no tomorrow !!!