[L 数组符号 - 它来自哪里? [英] [L array notation - where does it come from?

问题描述

我经常看到使用 [L 和一个类型来表示数组的消息，例如:

[Ljava.lang.Object;不能强制转换为 [Ljava.lang.String;

(以上是我刚刚抽出的一个任意示例.)我知道这表示一个数组，但是语法从何而来?为什么以 [ 开头但没有结束方括号?为什么是L?这纯粹是随意的还是背后有其他历史/技术原因?

解决方案

[ 代表 Array，Lsome.type.Here 代表类型.这类似于

I've often seen messages that use [L then a type to denote an array, for instance:

[Ljava.lang.Object; cannot be cast to [Ljava.lang.String;

(The above being an arbitrary example I just pulled out.) I know this signifies an array, but where does the syntax come from? Why the beginning [ but no closing square bracket? And why the L? Is it purely arbitrary or is there some other historical/technical reason behind it?

解决方案

[ stands for Array, the Lsome.type.Here means the type. That's similar to the type descriptors used internally in the bytecode seen in §4.3 of the Java Virtual Machine Specification -- picked to be as brief as possible. The only difference is in that the real descriptors use / rather than . for denoting packages.

For instance, for primitives the value is: [I for array of int, a two-dimensional array would be: [[I.

Since classes may have any name, it would be harder to identify what class it is, hence the L, the class name ends with a ;

Descriptors are also used to represent the types of fields and methods.

For instance:

(IDLjava/lang/Thread;)Ljava/lang/Object;

... corresponds to a method whose parameters are int, double, and Thread and the return type is Object

edit

You can also see this in .class files using the java dissambler

C:>more > S.java
class S {
  Object  hello(int i, double d, long j, Thread t ) {
   return new Object();
  }
}
^C
C:>javac S.java

C:>javap -verbose S
class S extends java.lang.Object
  SourceFile: "S.java"
  minor version: 0
  major version: 50
  Constant pool:
const #1 = Method       #2.#12; //  java/lang/Object."<init>":()V
const #2 = class        #13;    //  java/lang/Object
const #3 = class        #14;    //  S
const #4 = Asciz        <init>;
const #5 = Asciz        ()V;
const #6 = Asciz        Code;
const #7 = Asciz        LineNumberTable;
const #8 = Asciz        hello;
const #9 = Asciz        (IDJLjava/lang/Thread;)Ljava/lang/Object;;
const #10 = Asciz       SourceFile;
const #11 = Asciz       S.java;
const #12 = NameAndType #4:#5;//  "<init>":()V
const #13 = Asciz       java/lang/Object;
const #14 = Asciz       S;

{
S();
  Code:
   Stack=1, Locals=1, Args_size=1
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   return
  LineNumberTable:
   line 1: 0


java.lang.Object hello(int, double, long, java.lang.Thread);
  Code:
   Stack=2, Locals=7, Args_size=5
   0:   new     #2; //class java/lang/Object
   3:   dup
   4:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   7:   areturn
  LineNumberTable:
   line 3: 0


}

And in raw class file ( look at line 5 ):

Reference: Field description on the JVM specification

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