Agent mode of Javaweb

I. Dynamic Agent 1.1, Proxy mode what is proxy mode and its function

Proxy pattern (i.e., agent mode), one of the 23 common object-oriented software design patterns

Proxy mode definition: Provides a proxy for other objects to control access to this object. In some cases, an object does not fit or cannot directly reference another object, and a proxy object can act as an intermediary between the client and the target object.

Advantages:

• (1). The role of clear and real responsibility is to achieve the actual business logic, do not care about other non-responsibility transactions, through the late agent to complete a complete transaction, the accompanying result is simple and clear programming.
• (2). The proxy object can act as an intermediary between the client and the target object, which plays a role in hiding and protecting the target object.
• (3). High scalability

Structure

• One is the real object you want to access (the target class), the other is the proxy object, the real object and the proxy
• Object implements the same interface, first accesses the proxy class and then accesses the object that is actually being accessed.

1.2. Dynamic Agent

Dynamic proxy It can generate a proxy object directly to a target object without the need for a proxy class to exist.

Dynamic agent and Proxy mode principle is the same, but it does not have a specific proxy class, directly through the reflection generated a proxy object.

Dynamic Agent Generation Technology:

1. The JDK provides a proxy class that can directly generate agent objects directly to the object implementing the interface class.
2. Cglib (Spring Learning)

The Java.lang.reflect.Proxy class can generate a proxy object directly

Proxy.newproxyinstance (): Generates an instance of the proxy class. A class that can only be proxied to implement at least one interface

• ClassLoader: Class loader. Fixed notation, and the same class loader used by the proxy class.
• Class[] Interface: The interface to be implemented by the proxy class. Fixed notation, and the same interface can be used by the proxy class.
• Invocationhandler: Application of the Policy (scheme) design pattern . How to proxy?
  • Invoke method in Nvocationhandler: Any method that invokes the proxy class, this method executes the
    • Object Proxy: A reference to the proxy object itself. Generally not.
    • Methods: The method that is currently being called.
    • object[] args: Parameters used by the current method

1.3, AOP programming ideas (slicing programming)

1. Problem: There will be a lot of business methods in the future, there will be many duplicate code

2. Question: There are already many methods that do not take into account the problems of the business, and now ask to add.

Second, note:

• Note It is not a comment comment is written by the programmer, to the programmer's
• Annotations are given to the program to describe how the program runs and at what stage.

Annotations now in real-world development, the biggest feature is to replace the configuration file. Annotations are a new feature of jdk1.5 and can be reflected to make annotations functional.

Annotation @xxxx

2.1. Custom Annotations 1, three basic annotations in JDK:

A, @Override: The check subclass is indeed a method that overrides the parent class.

B, @Deprecated: The explanation is out of date.

C, @SuppressWarnings ({"Unused", "deprecation"}): Suppresses warnings in the program. The type of unused warning. {} array. All suppresses all warnings.

@SuppressWarnings ("All")

2. Syntax for custom annotations: (physical)

Look at the nature of annotations

Declares an annotation @interface note name {}

Public @interface myannotation{}

Note Its essence is an interface, which needs to inherit the annotation interface.

Public interface Myannotation extends Java.lang.annotation.Annotation {

}

Analyze members in annotations

Annotations are essentially interfaces and can have attribute methods in interfaces

Properties: Example: int age ();

What are the attribute types for annotations?

1. Basic type
2. String
3. Enum type
4. Annotation type
5. Class type
6. One-dimensional array type of the above types

Note: A tag is added to a location in your program code.

3. Reflection of Annotations: (Soul)

A, reflective annotation class

Java.lang.reflect.AnnotatedElement:

1. <t extends annotation> T getannotation (class<t> annotationtype): Gets a note reference of the specified type. No null returned.
2. Annotation[] Getannotations (): Gets all the annotations, including inherited from the parent class.
3. Annotation[] Getdeclaredannotations (): Get annotations on yourself.
4. Boolean isannotationpresent (class<? extends Annotation> Annotationtype): Determines if the specified annotation is not.

Class, Method, Field, constructor, etc. implement the Annotatedelement interface.

such as: Class.isannotationpresent (Mytest.class): Judgment class above there is no @mytest annotation;

Method.isannotationpresent (Mytest.class): There is no @mytest annotation on the judging method.

B. Properties in reflection annotations

4, Yuan annotation

A, custom annotations survival range (life cycle): Default is class.

What are meta annotations:

Annotations that can only be used on annotations are called meta annotations. (that is, annotations used to modify annotations)

@Retention: role. Changes the survival scope of the custom annotations.

• Retentionpolicy:
• SOURCE
• CLASS
• RUNTIME

@Target: The function that specifies where the annotation can be used.

• ElementType:
• TYPE:
• METHOD:
• FIELD:
• Annotation_type

@Documented: The function, the class using the @mytest annotation, if the @mytest annotations have @documented annotations above, then the @mytest annotation of the class's API document will appear @mytest figure.

@Inherited: A function that indicates that the annotation can be inherited.

Brief introduction of several annotations in Servlet3.0

Added support for annotations.

Servlet3.0:

tomcat7+;jdk6.0+;

Servlet3.0:web.xml is not a must. the alternative is annotations.

Four, class loader

1, function: Responsible for loading the class file on the disk into the JVM, class reference byte code

2. The class loader in the JVM:

1. BootStrap: Boss. The ancestor of the class loader. Printing it will get null.
   • Responsible for loading Jre/lib/rt.jar (most of the classes in the JDK)
2. Extclassloader: Responsible for loading Jre/lib/ext/*.jar
3. Appclassloader: All classes that are responsible for loading in the CLASSPATH environment variable.

3. Parent class delegation mechanism

Class A extends HttpServlet find A and then look up (the classes you write will be placed under the classpath path)

Agent mode of Javaweb