java 自定义类加载器ClassLoader的实现方式

深藏阁楼爱情的钟 2022-07-13 04:47 187阅读 0赞

一：继承ClassLoader

package com.lean.classload;
    
    import java.io.ByteArrayOutputStream;
    import java.io.File;
    import java.io.FileInputStream;
    import java.io.IOException;
    import java.io.InputStream;
    
    /**
     * 自定义路径类加载器
     * 继承ClassLoader
     *
     */
    public class PathClassLoader extends ClassLoader{
    	private String classPath; //类的路径
    	private final String packageName = "net.";//包名
    	
    	
    	/* 重写加载类的方法
    	 * (non-Javadoc)
    	 * @see java.lang.ClassLoader#findClass(java.lang.String)
    	 */
    	@Override
    	protected java.lang.Class<?> findClass(String name) throws ClassNotFoundException {
    		if(packageName.startsWith(name)){
    			byte [] classData = this.getData(name);
    			if(null == classData){
    				throw new ClassNotFoundException(name);
    			}else{
    				//Converts an array of bytes into an instance of 
    				//class Class. Before the Class can be used it must 
    				//be resolved. This method assigns 
    				return super.defineClass(name,classData,0,classData.length);
    			}
    		}else{
    			return super.loadClass(name);
    		}
    	}
    	
    	/**
    	 * 从文件中获取字节流
    	 * @param className
    	 * @return
    	 */
    	private byte[] getData(String className){
    		//File.separator  解决系统分隔符问题
    		String path= classPath + File.separator +className.replace(".", File.separator) +".class";
    		try {
    			InputStream is = new FileInputStream(path);
    			ByteArrayOutputStream stream = new ByteArrayOutputStream();
    			byte [] buffer = new byte[1048];
    			int num = 0;
    			while((num = is.read(buffer)) != -1){
    				stream.write(buffer, 0, num);
    			}
    			return stream.toByteArray();
    		} catch (IOException e) {
    			// TODO Auto-generated catch block
    			e.printStackTrace();
    		}
    		return null;
    	}
    	
    	public String getClassPath() {
    		return classPath;
    	}
    
    	public void setClassPath(String classPath) {
    		this.classPath = classPath;
    	}
    }

二：继承URLClassLoader

package com.lean.classload;
    
    import java.net.URL;
    import java.net.URLClassLoader;
    
    /**
     * * 继承URLClassLoader
     *
     */
    public class URLPathClassLoader extends URLClassLoader {
    	private final String packageName = "net.";// 包名
    
    	public URLPathClassLoader(URL[] urls, ClassLoader classLoader) {
    		super(urls, classLoader);
    		// TODO Auto-generated constructor stub
    	}
    
    	@Override
    	protected Class<?> findClass(String name) throws ClassNotFoundException {
    		Class<?> clazz = findLoadedClass(name);
    		if (null != clazz) {
    			return clazz;
    		}
    		if (packageName.startsWith(name)) {
    			return super.findClass(name);
    		} else {
    			return super.loadClass(name);
    		}
    	}
    }