今天在看到《Java疯狂讲义》中一个章节习题：

开发一个工具类，该工具类提供一个eval()方法，实现JavaScript中eval()函数的功能——可以动态运行一行或多行程序代码。例如：eval("System.out.println("hello world")")，将输出hello world。

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code:

import java.io.*;

/**

 * Java创建一个类似于js中eval()的方法

 */

public class EvalUtil {

    public static void main(String[] args) throws Exception {

        eval("int a = c; int b = 2; System.out.println(a+b);");

    }

    public static void eval(String javaStr) throws Exception {

        StringBuffer str = new StringBuffer();

        str.append("public class Eval {").append("public static void main(String[] args) {").append(javaStr).append("}}");

        OutputStream out = new FileOutputStream("Eval.java");

        out.write(str.toString().getBytes("gbk"));

        out.close();

        // 编译时  错误信息

        Process javacProcess = Runtime.getRuntime().exec("javac Eval.java");

        InputStream compileError = javacProcess.getErrorStream();

        System.err.println(read(compileError));

        compileError.close();

        Process javaProcess = Runtime.getRuntime().exec("java Eval");

        //运行时   错误信息

        InputStream err = javaProcess.getErrorStream();

        System.err.println(read(err));

        err.close();

        //运行时   正常信息

        InputStream success = javaProcess.getInputStream();

        System.out.println(read(success));

        success.close();

        //删除生成文件

        new File("Eval.java").delete();

        new File("Eval.class").delete();

    }

    private static String read(InputStream in) throws IOException {

        byte[] b = new byte[1024];

        int len = -1;

        StringBuffer str = new StringBuffer();

        while ((len = in.read(b)) != -1) {

            str.append(new String(b, 0, len, "gbk"));

        }

        return str.toString();

    }

}