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
Process finished with exit code 0
不同:maven的多了一个-classpath参数!
结论:第一种java工程,idea没有指定classpath参数,所以肯定找不到要执行的类,不知道是我配置问题,还是这就是个idea的bug,这个是问题?,我推荐使用第二种maven工程。