JVM（五），ClassLoader

5、 ClassLoader

1.什么是ClassLoader

 

2.四种ClassLoader

 

3.自定义CLassLoader

 

（1）MyClassLoader

public class MyClassLoader extends ClassLoader {
    private String path;
    private String classLoaderName;

    public MyClassLoader(String path, String classLoaderName) {
        this.path = path;
        this.classLoaderName = classLoaderName;
    }

    //用于寻找类文件
    @Override
    public Class findClass(String name) {
        byte[] b = loadClassData(name);
        return defineClass(name, b, 0, b.length);
    }

    //用于加载类文件
    private byte[] loadClassData(String name) {
        name = path + name + ".class";
        InputStream in = null;
        ByteArrayOutputStream out = null;
        try {
            in = new FileInputStream(new File(name));
            out = new ByteArrayOutputStream();
            int i = 0;
            while ((i = in.read()) != -1) {
                out.write(i);
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                out.close();
                in.close();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        return out.toByteArray();
    }
}

原理仍是经过ClassLoader中的deFineClass方法来获取Class类型对象，自定义的是路径

（2）实现ClassLoaderChecker

public class ClassLoaderChecker {
    public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException {
        MyClassLoader m = new MyClassLoader("/Users/baidu/Desktop/", "myClassLoader");
        Class c = m.loadClass("Wali");
        c.newInstance();
    }
}

 （3）实现效果

4.类加载器的双亲委派机制

 

为何要使用双亲委派机制来加载class文件-避免多份一样的字节码的加载

5.类的加载方式

 

隐式加载：new

显示加载：loadClass，formName

（1）类装载过程

 

（2）代码实例区别

 

public class LoadDifference {
    public static void main(String[] args) throws Exception {
        //loadClass加载类，须要调用c.newInstance()才会加载类
        ClassLoader cl = Robot.class.getClassLoader();
        Class c = cl.loadClass("com.interview.javabasic.reflect.Robot");
        c.newInstance();
        //forName加载类，在加载类的时候会将Static静态代码块的代码实现出来
        Class r = Class.forName("com.interview.javabasic.reflect.Robot");
    }
}

  使用Class.forName(classname)才能在反射回去类的时候执行static块。（3）数据库连接为何使用Class.forName(className)