JVM（五），ClassLoader

5、 ClassLoader

1.什麼是ClassLoader

 

2.四種ClassLoader

 

3.自定義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;
    }

    //用於尋找類文件
    @Override
    public Class findClass(String name) {
        byte[] b = loadClassData(name);
        return defineClass(name, b, 0, b.length);
    }

    //用於加載類文件
    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();
    }
}

原理仍是經過ClassLoader中的deFineClass方法來獲取Class類型對象，自定義的是路徑

（2）實現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）實現效果

4.類加載器的雙親委派機制

 

爲何要使用雙親委派機制來加載class文件-避免多份一樣的字節碼的加載

5.類的加載方式

 

隱式加載：new

顯示加載：loadClass，formName

（1）類裝載過程

 

（2）代碼實例區別

 

public class LoadDifference {
    public static void main(String[] args) throws Exception {
        //loadClass加載類，須要調用c.newInstance()纔會加載類
        ClassLoader cl = Robot.class.getClassLoader();
        Class c = cl.loadClass("com.interview.javabasic.reflect.Robot");
        c.newInstance();
        //forName加載類，在加載類的時候會將Static靜態代碼塊的代碼實現出來
        Class r = Class.forName("com.interview.javabasic.reflect.Robot");
    }
}

  使用Class.forName(classname)才能在反射回去類的時候執行static塊。（3）數據庫連接爲何使用Class.forName(className)