类加载器

类加载器就是加载class文件，获取二进制字节流。

类加载过程源码

步骤大概是

检查该类是否已经加载过了
如果有父类，就用父类loadClass
父类loadClass失败，就调用自己的findClass

package java.lang;

public Class<?> loadClass(String name) throws ClassNotFoundException {
        return loadClass(name, false);
}

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);//检查是否已经加载过了
        if (c == null) {
            long t0 = System.nanoTime();
            try {
                if (parent != null) {//有父就使用父类加载器
                    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) {//父类加载不到就是用findClass子类自己的加载器
                // 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;
    }
}

不使用双亲委派

重写loadClass方法，就可以实现

class MyClassLoader extends ClassLoader{
    @Override
    public Class<?> loadClass(String name) throws ClassNotFoundException {
        System.out.println("myClassLoader load");
        return loadClass(name, false);
    }
    
}

使用双亲委派

重写findClass方法就可以实现

class MyClassLoader extends ClassLoader{
    @Override
    public Class<?> findClass(String name) throws ClassNotFoundException {
        System.out.println("myClassLoader load");
        return loadClass(name, false);
    }   
}