我在String中编写了一个简单的Java代码,想在程序运行时动态执行,以下是我的代码：

import java.io.File;
import java.io.IOException;
import java.lang.reflect.Method;
import java.net.URL;
import java.net.URLClassLoader;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;

import javax.tools.JavaCompiler;
import javax.tools.ToolProvider;

public class CompliedClass {
    public static void main(String[] args) {
        String source = ""
                +"class Solution{"
                +"public int add(){"
                +"return 1+1;"
                +"}"
                +"}";

        //File root = new File("");
        File sourceFile = new File("./src/Solution.java");
        try {
            Files.write(sourceFile.toPath(), source.getBytes(StandardCharsets.UTF_8));
        } catch (IOException e) {
            e.printStackTrace();
        }
        System.out.println(sourceFile.getPath());
        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
        if (compiler == null) {
            System.out.println("JDK required (running inside of JRE)");
        }else{
            System.out.println("you got it!");
        }

        int compilationResult = compiler.run(null, null, null,sourceFile.getPath());
        if(compilationResult == 0){
            System.out.println("Compilation is successful");
        }else{
            System.out.println("Compilation Failed");
        }

        try{
            URLClassLoader classLoader = URLClassLoader.newInstance(new URL[] { sourceFile.toURI().toURL() });
            Class<?> cls = Class.forName("Solution" , true, classLoader);
            Object instance = cls.newInstance();
            Method method = cls.getDeclaredMethod("add", null);
            System.out.println(method.invoke(instance, null));
        }catch(Exception e){
            System.out.println("something wrong");
        }

    }
}

上面的代码的问题是当我第一次执行时,我无法获得结果,似乎下面的代码有一个例外：

Object instance = cls.newInstance();

然后我第二次执行,它的功能很好,所以结论是当我第一次运行时,找不到Solution类,这导致下面的异常

java.lang.ClassNotFoundException: Solution

有人可以帮我解决这个问题吗？

解决方法:

1)您的类应具有public修饰符,否则,如果未在同一包中声明,CompliedClass将无法访问它.在这里它可以像默认包一样工作,但是不建议这样做.

2)您的错误在这里：

URLClassLoader classLoader = URLClassLoader.newInstance(new URL[] { sourceFile.toURI().toURL() });

这里的sourceFile是指解决方案java类文件.
而是应根据URLClassLoader的javadoc java.net.URLClassLoader.newInstance(URL [] urls)(即url参数)引用包含Solution Java类文件的文件夹的url.
是指用于搜索类和资源的URL.

您可以尝试使用以下两个经过解释的修改来编写此代码：

public class CompliedClass {
    public static void main(String[] args) {

        String source = "public class Solution{" + "public int add(){" + "return 1+1;" + "}" + "}";

        File folder = new File("./src");
        File sourceFile = new File(folder, "Solution.java");

        try {
            Files.write(sourceFile.toPath(), source.getBytes(StandardCharsets.UTF_8));
        } catch (IOException e) {
            e.printStackTrace();
        }
        System.out.println(sourceFile.getPath());
        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
        if (compiler == null) {
            System.out.println("JDK required (running inside of JRE)");
        } else {
            System.out.println("you got it!");
        }

        int compilationResult = compiler.run(null, null, null, sourceFile.getPath());
        if (compilationResult == 0) {
            System.out.println("Compilation is successful");
        } else {
            System.out.println("Compilation Failed");
        }

        try {
            URLClassLoader classLoader = URLClassLoader.newInstance(new URL[] {folder.toURI().toURL() });
            Class<?> cls = Class.forName("Solution", true, classLoader);
            Object instance = cls.newInstance();
            Method method = cls.getDeclaredMethod("add", null);
            System.out.println(method.invoke(instance, null));
        } catch (Exception e) {
            e.printStackTrace();
            System.out.println("something wrong");
        }

    }
}

输出：

.\src\Solution.java

you got it!

Compilation is successful

2