深入探讨 Java 类加载器

 

 

实现自定义类加载器，需要继承ClassLoader类，重写findClass方法即可：

 

1. 例如下面是一个加载其它地方的class文件的自定义类加载器：

 

package classloader;

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

public class FileSystemClassLoader extends ClassLoader { 

    private String rootDir; 

    /**
     * 指定自定义类加载器的搜索根路径（要指定为非当前classpath的路径）
     */
    public FileSystemClassLoader(String rootDir) { 
        this.rootDir = rootDir; 
    } 

    protected Class<?> findClass(String name) throws ClassNotFoundException { 
        byte[] classData = getClassData(name); 
        if (classData == null) { 
            throw new ClassNotFoundException(); 
        } 
        else { 
            return defineClass(name, classData, 0, classData.length); 
        } 
    } 

    private byte[] getClassData(String className) { 
        String path = classNameToPath(className); 
        try { 
            InputStream ins = new FileInputStream(path); 
            ByteArrayOutputStream baos = new ByteArrayOutputStream(); 
            int bufferSize = 4096; 
            byte[] buffer = new byte[bufferSize]; 
            int bytesNumRead = 0; 
            while ((bytesNumRead = ins.read(buffer)) != -1) { 
                baos.write(buffer, 0, bytesNumRead); 
            } 
            return baos.toByteArray(); 
        } catch (IOException e) { 
            e.printStackTrace(); 
        } 
        return null; 
    } 

    private String classNameToPath(String className) { 
        return rootDir + File.separatorChar 
                + className.replace('.', File.separatorChar) + ".class"; 
    } 
 }

 

然后测试一下：

 

public static void main(String[] args) throws Exception {
		//getStatisticResult();
		FileSystemClassLoader loader = new FileSystemClassLoader("D:\\ws\\2013-12-price\\tt\\bin");
		Class<?> clazz = loader.loadClass("com.tch.test.classloader.Student");
		System.out.println(clazz.getClassLoader());
		System.out.println(clazz.getMethods()[0].getName());
	}

 

通过打印clazz的classloader可以知道使用的是我们自定义的类加载器

 

 

2. 从网络加载class（跟上面差别不大，只是获取字节码的方式不同，通过url打开流，然后读取和加载字节码）：

 

package classloader;

import java.io.ByteArrayOutputStream;
import java.io.InputStream;
import java.net.URL;

public class NetworkClassLoader extends ClassLoader {
	
	private String rootUrl;
	
	public NetworkClassLoader(String rootUrl) {
		this.rootUrl = rootUrl;
	}

	protected Class<?> findClass(String name) throws ClassNotFoundException {
		byte[] classData = getClassData(name);
		if (classData == null) {
			throw new ClassNotFoundException();
		}
		else {
			return defineClass(name, classData, 0, classData.length);
		}
	}
	
	private byte[] getClassData(String className) {
		String path = classNameToPath(className);
		try {
			URL url = new URL(path);
			InputStream ins = url.openStream();
			ByteArrayOutputStream baos = new ByteArrayOutputStream();
			int bufferSize = 4096;
			byte[] buffer = new byte[bufferSize];
			int bytesNumRead = 0;
			while ((bytesNumRead = ins.read(buffer)) != -1) {
				baos.write(buffer, 0, bytesNumRead);
			}
			return baos.toByteArray();
		} catch (Exception e) {
			e.printStackTrace();
		}
		return null;
	}
	
	private String classNameToPath(String className) {
		return rootUrl + "/"
				+ className.replace('.', '/') + ".class";
	}
}