自定义类加载器（2）

1.基本介绍
JVM认为同一个类的2个条件，第一相同的类加载器引用，第二相同的名称(包名+类名)。也就是说，同一个.class文件被不同的类加载器加载，却不是相同的类。

2.自定义类加载器方式

如果不想打破双亲委派模型, 那么只需要重写findClass方法即可
如果想打破双亲委派模型，那么就重写整个loadClass方法

3.不打破双亲委派模型实现

public class CustomAClassLoader extends ClassLoader {
    private String filePath = null;

    protected synchronized Class<?> findClassByFilePathAndName(String filePath, String name)
            throws ClassNotFoundException {
        if (Objects.isNull(filePath) || Objects.isNull(name)) {
            throw new IllegalArgumentException("参数不能为空");
        }
        this.filePath = filePath;
        return findClass(name);
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        StringBuilder stringBuilder = new StringBuilder();
        stringBuilder.append(filePath)
                .append(name.replace(".", "/"))
                .append(".class");
        FileInputStream fis = null;
        ByteArrayOutputStream baos = null;
        try {
            baos = new ByteArrayOutputStream();
            fis = new FileInputStream(new File(stringBuilder.toString()));
            byte[] bytes = new byte[1024];
            int len = -1;
            while ((len = fis.read(bytes)) != -1) {
                baos.write(bytes, 0, len);
            }
            return this.defineClass(name, baos.toByteArray(), 0, baos.size());
        } catch (Exception e) {
            throw new ClassNotFoundException();
        } finally {
            closeStream(fis);
            closeStream(baos);
        }
    }

    private void closeStream(InputStream stream) {
        if (Objects.nonNull(stream)) {
            try {
                stream.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

    private void closeStream(OutputStream stream) {
        if (Objects.nonNull(stream)) {
            try {
                stream.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

4.自定义被加载的类

public interface IUserDao {
    void save();
}

public class UserDao implements IUserDao {
    @Override
    public void save() {
        System.out.println("save method.");
    }
}

5.测试

public static void main(String[] args) throws Exception {
    String name = "test.UserDao";
    String filePath = "D:\\test\\target\\classes\\";
    CustomAClassLoader classLoader = new CustomAClassLoader();
    Class<IUserDao> userDaoClass = (Class<IUserDao>) classLoader.findClassByFilePathAndName(filePath, name);
    IUserDao userDao = userDaoClass.newInstance();
    userDao.save();


    Class<IUserDao> appUserDaoClass = (Class<IUserDao>) ClassLoaderDemo.class.getClassLoader().loadClass(name);
    IUserDao appUserDao = appUserDaoClass.newInstance();
    appUserDao.save();

    while (true) {

    }
}

6.分析其在JVM中，存在几个class
在这里插入图片描述

自定义类加载器（2）

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