为什么在Java中这么难做到这一点?如果您希望拥有任何类型的模块系统,您需要能够动态加载JAR文件。有人告诉我,有一种方法可以通过编写自己的ClassLoader来实现,但这需要做很多工作,因为(至少在我看来)应该像调用以JAR文件作为参数的方法一样简单。
对实现这一功能的简单代码有什么建议吗?
为什么在Java中这么难做到这一点?如果您希望拥有任何类型的模块系统,您需要能够动态加载JAR文件。有人告诉我,有一种方法可以通过编写自己的ClassLoader来实现,但这需要做很多工作,因为(至少在我看来)应该像调用以JAR文件作为参数的方法一样简单。
对实现这一功能的简单代码有什么建议吗?
当前回答
我个人认为java.util.ServiceLoader做得很好。你可以在这里得到一个例子。
其他回答
另一个来自Allain的黑客解决方案版本,也适用于JDK 11:
File file = ...
URL url = file.toURI().toURL();
URLClassLoader sysLoader = new URLClassLoader(new URL[0]);
Method sysMethod = URLClassLoader.class.getDeclaredMethod("addURL", new Class[]{URL.class});
sysMethod.setAccessible(true);
sysMethod.invoke(sysLoader, new Object[]{url});
在JDK 11上,它给出了一些弃用警告,但对于那些在JDK 11上使用Allain解决方案的人来说,它只是一个临时解决方案。
我找到的最好的是org.apache.xbean.classloader.JarFileClassLoader,它是XBean项目的一部分。
下面是我过去使用的一个简短方法,用于从特定目录中的所有lib文件创建一个类装入器
public void initialize(String libDir) throws Exception {
File dependencyDirectory = new File(libDir);
File[] files = dependencyDirectory.listFiles();
ArrayList<URL> urls = new ArrayList<URL>();
for (int i = 0; i < files.length; i++) {
if (files[i].getName().endsWith(".jar")) {
urls.add(files[i].toURL());
//urls.add(files[i].toURI().toURL());
}
}
classLoader = new JarFileClassLoader("Scheduler CL" + System.currentTimeMillis(),
urls.toArray(new URL[urls.size()]),
GFClassLoader.class.getClassLoader());
}
然后使用类加载器,只需执行:
classLoader.loadClass(name);
虽然这里列出的大多数解决方案要么是hack(在JDK 9之前)难以配置(代理),要么就是不再工作(在JDK 9之后),但我发现没有人提到一个清晰的文档化方法,这真的很令人惊讶。
您可以创建一个自定义系统类装入器,然后您可以自由地做任何您想做的事情。不需要反射,所有类共享相同的类加载器。
当启动JVM时添加这个标志:
java -Djava.system.class.loader=com.example.MyCustomClassLoader
类加载器必须有一个接受类加载器的构造函数,必须将类加载器设置为父类加载器。构造函数将在JVM启动时调用,真正的系统类加载器将被传递,主类将由自定义加载器加载。
要添加jar,只需调用ClassLoader.getSystemClassLoader()并将其转换为您的类。
请查看此实现,以获得精心制作的类装入器。请注意,您可以将add()方法更改为public。
我知道我来晚了,但我一直在使用pf4j,这是一个插件框架,它工作得非常好。
PF4J是一个微框架,其目标是保持核心简单但可扩展。
一个插件使用的例子:
在你的应用/插件中使用ExtensionPoint接口标记定义一个扩展点:
public interface Greeting extends ExtensionPoint {
String getGreeting();
}
使用@Extension注释创建一个扩展:
@Extension
public class WelcomeGreeting implements Greeting {
public String getGreeting() {
return "Welcome";
}
}
然后你可以加载和卸载插件,如你所愿:
public static void main(String[] args) {
// create the plugin manager
PluginManager pluginManager = new JarPluginManager(); // or "new ZipPluginManager() / new DefaultPluginManager()"
// start and load all plugins of application
pluginManager.loadPlugins();
pluginManager.startPlugins();
// retrieve all extensions for "Greeting" extension point
List<Greeting> greetings = pluginManager.getExtensions(Greeting.class);
for (Greeting greeting : greetings) {
System.out.println(">>> " + greeting.getGreeting());
}
// stop and unload all plugins
pluginManager.stopPlugins();
pluginManager.unloadPlugins();
}
欲了解更多细节,请参阅文档
下面是一个未弃用的版本。我修改了原始版本,删除了不推荐使用的功能。
/**************************************************************************************************
* Copyright (c) 2004, Federal University of So Carlos *
* *
* All rights reserved. *
* *
* Redistribution and use in source and binary forms, with or without modification, are permitted *
* provided that the following conditions are met: *
* *
* * Redistributions of source code must retain the above copyright notice, this list of *
* conditions and the following disclaimer. *
* * Redistributions in binary form must reproduce the above copyright notice, this list of *
* * conditions and the following disclaimer in the documentation and/or other materials *
* * provided with the distribution. *
* * Neither the name of the Federal University of So Carlos nor the names of its *
* * contributors may be used to endorse or promote products derived from this software *
* * without specific prior written permission. *
* *
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS *
* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT *
* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR *
* A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR *
* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, *
* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, *
* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR *
* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF *
* LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING *
* NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS *
* SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. *
**************************************************************************************************/
/*
* Created on Oct 6, 2004
*/
package tools;
import java.io.File;
import java.io.IOException;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.net.URL;
import java.net.URLClassLoader;
/**
* Useful class for dynamically changing the classpath, adding classes during runtime.
*/
public class ClasspathHacker {
/**
* Parameters of the method to add an URL to the System classes.
*/
private static final Class<?>[] parameters = new Class[]{URL.class};
/**
* Adds a file to the classpath.
* @param s a String pointing to the file
* @throws IOException
*/
public static void addFile(String s) throws IOException {
File f = new File(s);
addFile(f);
}
/**
* Adds a file to the classpath
* @param f the file to be added
* @throws IOException
*/
public static void addFile(File f) throws IOException {
addURL(f.toURI().toURL());
}
/**
* Adds the content pointed by the URL to the classpath.
* @param u the URL pointing to the content to be added
* @throws IOException
*/
public static void addURL(URL u) throws IOException {
URLClassLoader sysloader = (URLClassLoader)ClassLoader.getSystemClassLoader();
Class<?> sysclass = URLClassLoader.class;
try {
Method method = sysclass.getDeclaredMethod("addURL",parameters);
method.setAccessible(true);
method.invoke(sysloader,new Object[]{ u });
} catch (Throwable t) {
t.printStackTrace();
throw new IOException("Error, could not add URL to system classloader");
}
}
public static void main(String args[]) throws IOException, SecurityException, ClassNotFoundException, IllegalArgumentException, InstantiationException, IllegalAccessException, InvocationTargetException, NoSuchMethodException{
addFile("C:\\dynamicloading.jar");
Constructor<?> cs = ClassLoader.getSystemClassLoader().loadClass("test.DymamicLoadingTest").getConstructor(String.class);
DymamicLoadingTest instance = (DymamicLoadingTest)cs.newInstance();
instance.test();
}
}