谈谈你对java的理解

平台无关性
GC
语言特性
面向对象
类库
异常处理

平台无关如何实现

java源码首先被编译成字节码,再由不同平台的jvm进行解析,java语言在不同平台上运行时不需要重新编译,java虚拟机在执行字节码时,把字节码转换为具体平台上的机器指令

为什么jvm不直接将源码解析成机器码去执行

每次执行需要重新编译,进行语法检查语义分析等,性能就会受影响
别的语言也可以解析成字节码

jvm如何加载.class文件

jvm主要由ClassLoader,RuntimeDataArea,ExecutionEngine,NativeInterface这四部分组成,它通过classloader将符合特定格式的class文件加载到内存,再通过executionEngine对命令进行解析,并提交给操作系统执行

谈谈反射

写一个例子


public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException, NoSuchFieldException {
       Class rc = Class.forName("com.test.basic.Robot");
       Robot r = (Robot) rc.newInstance();
       System.out.println("class name is :" + rc.getName());

       Method getHello = rc.getDeclaredMethod("throwHello", String.class);
       getHello.setAccessible(true);
       Object str = getHello.invoke(r, "bob");
       System.out.println("get hello result:" + str);

       Method sayHi = rc.getMethod("sayHi", String.class);
       sayHi.invoke(r, "welcom");

       Field name = rc.getDeclaredField("name");
       name.setAccessible(true);
       name.set(r, "dd");
       sayHi.invoke(r, "welcome");

   }

类从编译到执行的过程

谈谈classloader

classloader的种类

自定义classloader的实现

关键函数

public class MyClassLoader extends ClassLoader {

    private String path;
    private String classLoaderName;

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

    @Override
    protected Class findClass(String name) {
        byte[] b = loadClassData(name);
        return defineClass(name, b, 0, b.length);
    }

    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 (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                in.close();
                out.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return out.toByteArray();
    }

}


public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException {
      MyClassLoader myClassLoader = new MyClassLoader("/home/zw/Documents/","Wali");
      Class c = myClassLoader.loadClass("Wali");
      System.out.println(c.getClassLoader());
      c.newInstance();

  }

谈谈类加载的双亲委派机制

为什么使用双亲委派机制

避免多份同样的字节码的加载

类加载的方式

隐式加载:new
显式加载:loadClass,forName等

loadClass和forName的区别

类的装载过程

区别:forName得到的class是完成初始化的,loadClass得到的class还没有链接

public static void main(String[] args) throws ClassNotFoundException {
//        ClassLoader cl = Robot.class.getClassLoader();
    Class c = Class.forName("com.test.basic.Robot");
}