idea 找不到或无法加载主类 java.lang.ClassNotFoundException

昨天研究类加载的双亲委派，按照demo敲敲加深记忆的时候，报了标题上的错。困扰了一夜，今天起来吃过饭就琢磨这个事情，刚刚终于搞明白了。

demo的目的是利用URLLoader加载磁盘上的Person类

我的demo的目录结构：

代码结构：

package com.classloader.test;

import java.io.File;
import java.net.MalformedURLException;
import java.net.URI;
import java.net.URL;
import java.net.URLClassLoader;

public class TestURLLoader {

    public static void main(String[] args) throws ClassNotFoundException, MalformedURLException, InstantiationException, IllegalAccessException {
        new LoaderFromDisk();
    }

}


class LoaderFromDisk {


    LoaderFromDisk() throws MalformedURLException, ClassNotFoundException, IllegalAccessException, InstantiationException {

        File file = new File("F:/git-worksapce/try-new-technologies/classLoader/src/com/world/");
        URI uri = file.toURI();
        URL url = uri.toURL();
        URLClassLoader urlClassLoader = new URLClassLoader(new URL[]{url});
        System.out.println("urlClassLoader的父加载器="+urlClassLoader.getParent());
        Class aclass = urlClassLoader.loadClass("com.world.Person");
        Object obj = aclass.newInstance();
    }
}

package com.world;


public class Person {
    private String name;
    private int age;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }


    @Override
    public String toString() {
        return "Person{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

很简单的demo，TestURLLoader中有main方法

我点击运行控制台的报错：

我们来看看上图红框中idea帮我们生成的执行类的命令：

"C:\Program Files\Java\java11\jdk-11.0.5\bin\java.exe" "-javaagent:D:\Program Files\JetBrains\IntelliJ IDEA 2019.3.1\lib\idea_rt.jar=51749:D:\Program Files\JetBrains\IntelliJ IDEA 2019.3.1\bin" -Dfile.encoding=UTF-8 com.classloader.test.TestURLLoader

上面的是idea创建的java project，

我们用idea创建maven工程，目录如下：

代码一模一样，只是这个是maven工程，我们run的结果：

成功了！！

我们分析idea两种方式，生成的运行命令：

方式一："C:\Program Files\Java\java11\jdk-11.0.5\bin\java.exe" "-javaagent:D:\Program Files\JetBrains\IntelliJ IDEA 2019.3.1\lib\idea_rt.jar=51749:D:\Program Files\JetBrains\IntelliJ IDEA 2019.3.1\bin" -Dfile.encoding=UTF-8 com.classloader.test.TestURLLoader
错误: 找不到或无法加载主类 com.classloader.test.TestURLLoader
原因: java.lang.ClassNotFoundException: com.classloader.test.TestURLLoader

Process finished with exit code 1



方式二：

"C:\Program Files\Java\jdk1.8.0_181\bin\java.exe" "-javaagent:D:\Program Files\JetBrains\IntelliJ IDEA 2019.3.1\lib\idea_rt.jar=52066:D:\Program Files\JetBrains\IntelliJ IDEA 2019.3.1\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_181\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_181\jre\lib\rt.jar;F:\git-worksapce\try-new-technologies\classLoader\target\classes" com.classloader.test.TestURLLoader
urlClassLoader的父加载器=sun.misc.Launcher$AppClassLoader@18b4aac2