JDK9可以像Unix脚本一样执行Java代码

现在你多少听说一些JDK9 JShell相关的内容，这篇文章中将介绍如何使用JShell像Unix脚本一样执行Java代码。

让我们开始一个简单的例子，再进行详细的介绍：

• 创建一个 java_shell_code.txt 的文件；

• 在文件中写一些Java代码，并执行以下命令。你没必须声明一个Class，你可以直接写一句代码，或者创建一个方法。

java ex.jshell.extension.JShellScriptExecutor ./java_shell_code.txt

java_shell_code.txt

// java_shell_code.txt contents.
/* If you observe it does not need a class declaration. Using the power of jshell, we can write functions and call functions with out creating class, just like functional programming */
 // You can also include comments like above, anywhere in the script
String var1 = "Hello";
System.out.println(var1);
public int getInt1(){
    return 2;
}
public int getInt2(){
    return 4;
}
getInt1() + getInt2();
public class MyClass{
    public void sayHelloWorld(){
        System.out.println("HelloWorld");
    }
}
new MyClass().sayHelloWorld()

执行结果：

"Hello"
Hello
6
HelloWorld

详细介绍

在JDK9中提供了一个新的类 JShell.java，它属于jdk.jshell模块。我们可以使用它执行Java代码片段，或创建一个Java方法，而不用创建一个类。如果这个Java代码片段中有错误，可以通过 Snippet.status状态（只有两种状态：REJECTED、VALID）来检查。

SourceCodeAnalysis是用来解析代码的，它使用分号、解析方法、或类的声明等。

下面是JShellScriptExecutor代码。你可以拷贝，并修改它。你也可以从GitHub仓库中克隆。

https://github.com/kotari4u/jshell_script_executor

package ex.jshell.extension;import jdk.jshell.JShell;import jdk.jshell.Snippet;import jdk.jshell.SnippetEvent;import jdk.jshell.SourceCodeAnalysis;import java.io.IOException;import java.nio.file.Files;import java.nio.file.Paths;import java.util.Objects;/** * This class can execute jshell expressions in sequence * We can write java commands like shell script and execute it. * Just write commands in a way that we give in jshell and save it in a file and execute it. * * @author Hemamabara Vamsi, Kotari * @since 5/27/2017. */public class JShellScriptExecutor {    public static void main(String[] args){        new JShellScriptExecutor().evaluate(args[0]);    }    public void evaluate(String scriptFileName){        try(JShell jshell = JShell.create()){          // Handle snippet events. We can print value or take action if evaluation failed.            jshell.onSnippetEvent(snippetEvent -> snippetEventHandler(snippetEvent));            String scriptContent = new String(Files.readAllBytes(Paths.get(scriptFileName)));            String s = scriptContent;            while (true) {                // Read source line by line till semicolon (;)                SourceCodeAnalysis.CompletionInfo an = jshell.sourceCodeAnalysis().analyzeCompletion(s);                if (!an.completeness().isComplete()) {                    break;                }                // If there are any method declaration or class declaration                 // in new lines, resolve it.                jshell.eval(trimNewlines(an.source()));                // EOF                if (an.remaining().isEmpty()) {                    break;                }                // If there is semicolon, execute next seq                s = an.remaining();            }        } catch (IOException e) {            e.printStackTrace();        }    }    public void snippetEventHandler(SnippetEvent snippetEvent){        String value = snippetEvent.value();        if(!Objects.isNull(value) && value.trim().length() > 0) {          // Prints output of code evaluation            System.out.println(value);        }        // If there are any erros print and exit        if(Snippet.Status.REJECTED.equals(snippetEvent.status())){            System.out.println("Evaluation failed : "+snippetEvent.snippet().toString()                                +"\nIgnoring execution of above script");        }    }    private String trimNewlines(String s) {        int b = 0;        while (b < s.length() && s.charAt(b) == '\n') {            ++b;        }        int e = s.length() -1;        while (e >= 0 && s.charAt(e) == '\n') {            --e;        }        return s.substring(b, e + 1);    }}

module-info.java

module ex.jshell.extension{     requires jdk.jshell;}

赶快试用下强大的JShell吧。