java 自定义classloader

java中默认提供了三类classloader，分别加载不同目录下的class或者jar包，如果有一些非通用的需求，比如想从一个特定的位置加载class，如http地址、网盘、U盘等；又比如想对class做一些隔离；这个时候，默认的classloader就不能满足要求了，需要自定义classloader

自定义classloader很简单，

（1）继承java.lang.ClassLoader

（2）重写父类相关的方法

先看下ClassLoader的相关代码，java.lang.ClassLoader#loadClass(java.lang.String, boolean)

protected Class<?> loadClass(String name, boolean resolve)
    throws ClassNotFoundException
{
    synchronized (getClassLoadingLock(name)) {
        // First, check if the class has already been loaded
        Class<?> c = findLoadedClass(name);
        if (c == null) {
            long t0 = System.nanoTime();
            try {
                if (parent != null) {
                    c = parent.loadClass(name, false);
                } else {
                    c = findBootstrapClassOrNull(name);
                }
            } catch (ClassNotFoundException e) {
                // ClassNotFoundException thrown if class not found
                // from the non-null parent class loader
            }

            if (c == null) {
                // If still not found, then invoke findClass in order
                // to find the class.
                long t1 = System.nanoTime();
                c = findClass(name);

                // this is the defining class loader; record the stats
                sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
                sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
                sun.misc.PerfCounter.getFindClasses().increment();
            }
        }
        if (resolve) {
            resolveClass(c);
        }
        return c;
    }
}

可以看到，jvm是根据一定的顺序来查找特定的class，如果父类或者bootstrapclassloader找不到，最终会调用findClass来加载指定的class，所以，只需重写该方法即可

网上的实例代码：https://blog.youkuaiyun.com/xyang81/article/details/7292380

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

/**
 * 加载网络class的ClassLoader
 */
public class NetworkClassLoader extends ClassLoader {

    private String rootUrl;

    public NetworkClassLoader(String rootUrl) {
        this.rootUrl = rootUrl;
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        Class clazz;
        //根据类的二进制名称,获得该class文件的字节码数组
        byte[] classData = getClassData(name);
        if (classData == null) {
            throw new ClassNotFoundException();
        }
        //将class的字节码数组转换成Class类的实例
        clazz = defineClass(name, classData, 0, classData.length); 
        return clazz;
    }

    private byte[] getClassData(String name) {
        InputStream is = null;
        try {
            String path = classNameToPath(name);
            URL url = new URL(path);
            byte[] buff = new byte[1024*4];
            int len = -1;
            is = url.openStream();
            ByteArrayOutputStream baos = new ByteArrayOutputStream();
            while((len = is.read(buff)) != -1) {
                baos.write(buff,0,len);
            }
            return baos.toByteArray();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            if (is != null) {
                try {
                    is.close();
                } catch(Exception e) {
                }
            }
        }
        return null;
    }

    private String classNameToPath(String name) {
        return rootUrl + "/" + name.replace(".", "/") + ".class";
    }

}