Java运行时动态加载类之ClassLoader

https://blog.csdn.net/fjssharpsword/article/details/64922083

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需求场景：动态加载类ClassLoaderd，在xml文件中配置加载类名称和方法，：

一、准备

1）在D:\\tmp\\目录下配置a.xml文件：

<?xml version="1.0" encoding="ISO-8859-1"?>
<classes>

 <class name="User">
 <method>say</method>
 </class> 

 <class name="map">
 <method>add</method>
 </class> 

</classes>

2）要动态加载的类：

package dx;

public class map {
    public void add(){
        System.out.println("1+1=2");
    }
}

package cn.fjs;

public class User {

    public void say(){
        System.out.println(" hello ...");
    }

}

对这两个类进行编译后，将class文件复制到D:\\tmp\\路径下。

二、参考代码如下

1、重载ClassLoader类：

package cn.fjs;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;

public class DynamicClassLoader extends ClassLoader{

    private static final String SUFFIX = ".class";
    public String[] paths;

    public DynamicClassLoader(String[] paths) {
        this.paths = paths;
    }

    public DynamicClassLoader(ClassLoader parent,String[] paths){
        super(parent);
        this.paths = paths;
    }

    @SuppressWarnings("deprecation")
    @Override
    protected Class<?> findClass(String className) throws ClassNotFoundException {
        String classPath = getClassPath(className);
        if(classPath != null){
            byte[] clazz = loadClazz(classPath);
            return defineClass(clazz, 0, clazz.length);
        }else{
            System.out.println("class is not found !");
            return null;
        }
    }

    public byte[] loadClazz(String classPath) {
        try {
            FileInputStream in = new FileInputStream(new File(classPath));
            ByteArrayOutputStream baos = new ByteArrayOutputStream();
            int b;
            while((b = in.read()) != -1){
                baos.write(b);
            }
            in.close();
            return baos.toByteArray();
        } catch (Exception e) {
            System.out.println(e);
        }
        return null;
    }

    public String getClassPath(String className){
        for(String path : paths){
            if(className.contains(".")){
                className = className.replaceAll(".", File.separator);
            }
            String classPath = path + className + SUFFIX;
            File classFile = new File(classPath);
            if(classFile.exists()){
                return classPath;
            }
        }
        return null;
    }
}

2、解析xml文件

package cn.fjs;

import java.io.IOException;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;

import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;
import org.xml.sax.SAXException;

//解析xml文件，获取类和方法
public class DynamicDom {
    private static DocumentBuilderFactory dbFactory = null;
    private static DocumentBuilder db = null;
    private static Document document = null;  

    static{
        try {
            dbFactory = DocumentBuilderFactory.newInstance();
            db = dbFactory.newDocumentBuilder();
        } catch (ParserConfigurationException e) {
            e.printStackTrace();
        }
    }  

    public Map<String,List<String>> getMethods(String fileName) throws SAXException, IOException{
        Map<String,List<String>> classes = new HashMap<String, List<String>>();
        document = db.parse(fileName);
        NodeList nList = document.getElementsByTagName("class");
        for(int i = 0 ; i<nList.getLength();i++){
            Node node = nList.item(i);
            Element ele = (Element)node;
            if(node.getNodeType() == Element.ELEMENT_NODE){
              String clazz = ele.getAttribute("name");
              List<String> methods = new ArrayList<String>();
              String method = ele.getElementsByTagName("method").item(0).getTextContent();
              methods.add(method);
              classes.put(clazz, methods);
            }
        }
        return classes;
    }
}

3、测试类：

package cn.fjs;

import java.util.List;
import java.util.Map;
import cn.fjs.DynamicClassLoader;
import cn.fjs.DynamicDom;

public class DynamicClassLoaderTest {
    public static void main(String[] args) {
        DynamicDom dmo = new DynamicDom();//xml文件解析类
        Map<String, List<String>> classes;
        //重载ClassLoader类
        DynamicClassLoader loader = new DynamicClassLoader(new String[]{"D:\\tmp\\"});
        try {
            classes = dmo.getMethods("D:\\tmp\\a.xml");
            for(String key:classes.keySet()){
                for(String clazz : classes.get(key)){
                    Class<?> c =loader.findClass(key);//类名字
                    c.getMethod(clazz).invoke(c.newInstance());//方法名字
                }
            }
        }catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

执行结果：

 hello ...
1+1=2

原文地址：https://www.cnblogs.com/zhao1949/p/9700159.html