概述

  1. JDK 9
  2. JShell 是 Java 的交互式编程环境
  3. JShell API 和工具提供了一种在 JShell 状态下评估 Java 编程语言的声明、语句和表达式的方式
  4. JShell 的状态包括不断发展的代码和执行状态
  5. 为了便于快速调查和编码
    • 语句和表达式不需要出现在方法中
    • 变量和方法也不需要出现在类中

启动 JShell

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$ jshell
|  Welcome to JShell -- Version 21.0.7
|  For an introduction type: /help intro

jshell>

详尽模式

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$ jshell -v
|  Welcome to JShell -- Version 21.0.7
|  For an introduction type: /help intro

jshell>

退出 JShell

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$ jshell -v
|  Welcome to JShell -- Version 21.0.7
|  For an introduction type: /help intro

jshell> /exit
|  Goodbye

JShell 命令

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$ jshell -v
|  Welcome to JShell -- Version 21.0.7
|  For an introduction type: /help intro

jshell> /help
|  Type a Java language expression, statement, or declaration.
|  Or type one of the following commands:
|  /list [<name or id>|-all|-start]
|      list the source you have typed
|  /edit <name or id>
|      edit a source entry
|  /drop <name or id>
|      delete a source entry
|  /save [-all|-history|-start] <file>
|      Save snippet source to a file
|  /open <file>
|      open a file as source input
|  /vars [<name or id>|-all|-start]
|      list the declared variables and their values
|  /methods [<name or id>|-all|-start]
|      list the declared methods and their signatures
|  /types [<name or id>|-all|-start]
|      list the type declarations
|  /imports
|      list the imported items
|  /exit [<integer-expression-snippet>]
|      exit the jshell tool
|  /env [-class-path <path>] [-module-path <path>] [-add-modules <modules>] ...
|      view or change the evaluation context
|  /reset [-class-path <path>] [-module-path <path>] [-add-modules <modules>]...
|      reset the jshell tool
|  /reload [-restore] [-quiet] [-class-path <path>] [-module-path <path>]...
|      reset and replay relevant history -- current or previous (-restore)
|  /history [-all]
|      history of what you have typed
|  /help [<command>|<subject>]
|      get information about using the jshell tool
|  /set editor|start|feedback|mode|prompt|truncation|format ...
|      set configuration information
|  /? [<command>|<subject>]
|      get information about using the jshell tool
|  /!
|      rerun last snippet -- see /help rerun
|  /<id>
|      rerun snippets by ID or ID range -- see /help rerun
|  /-<n>
|      rerun n-th previous snippet -- see /help rerun
|
|  For more information type '/help' followed by the name of a
|  command or a subject.
|  For example '/help /list' or '/help intro'.
|
|  Subjects:
|
|  intro
|      an introduction to the jshell tool
|  keys
|      a description of readline-like input editing
|  id
|      a description of snippet IDs and how use them
|  shortcuts
|      a description of keystrokes for snippet and command completion,
|      information access, and automatic code generation
|  context
|      a description of the evaluation context options for /env /reload and /reset
|  rerun
|      a description of ways to re-evaluate previously entered snippets

jshell>

立即执行的语句

使用 JShell 来评估 Java 语言的语句

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jshell> System.out.println("Hello, Rust");
Hello, Rust

jshell>

可覆盖的声明

支持变量的重复声明和类型变更 - JShell 是一个有状态的工具

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jshell> String greeting;
greeting ==> null
|  created variable greeting : String

jshell> String greeting = "Rust";
greeting ==> "Rust"
|  modified variable greeting : String
|    update overwrote variable greeting : String

jshell> System.out.println(greeting);
Rust
  
jshell> Integer greeting;
greeting ==> null
|  replaced variable greeting : Integer
|    update overwrote variable greeting : String

在可编译的代码里，在一个变量的作用域内，这个变量的类型是不允许转变的，也不允许重复声明的

独立的表达式

在 Java 程序中，语句是最小的可执行单位，表达式并不能单独存在 - JShell 支持表达式的输入

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jshell> 1 << 10
$7 ==> 1024
|  created scratch variable $7 : int

jshell> "Rust" == "Rust"
$8 ==> true
|  created scratch variable $8 : boolean

jshell> "Rust" == new String("Rust")
$9 ==> false
|  created scratch variable $9 : boolean