15. Structural Design Pattern-proxy pattern)

• Definition

Provide a proxy or a place for other objects to control access to this object. When the customer initiates a request to the proxy object for the first time, the proxy instantiates the real object and sends the request to it. In the future, all client requests are sent to the encapsulated real object through the proxy.

There are four proxy modes: Remote and virtual proxy modes:

1. Virtual Proxy: the proxy will not generate a real consumption proxy until it is necessary (first request ). It is used to store real objects that take a long time for an instance.

2. remote proxy: The local proxy object controls a remote object.

3. Protection proxy (Security Proxy): the proxy checks the permissions required to call real objects.

4. Smart reference: when calling a real object, the proxy handles other things.

(1) calculate the number of references of a real object and release it automatically when no reference is provided.

(2) objects that are referenced for the first time (stored on hard disk, memory, database, etc.

(3) check whether the real object is locked during access to ensure that no other object will change it.

The UML class diagram is as follows:

The relationships between classes and objects are as follows:

1. proxy: maintain a reference so that the proxy can access the object. If the realsubject and subject interfaces are the same, the proxy will reference subject; provide the same interface as the subject interface, the proxy can be used to replace objects. A bit like the adapter mode; controls access to the object and is responsible for creating and deleting it; other functions depend on the proxy type.

* Remote proxy: It is responsible for encoding requests and their parameters, and sending encoded requests to entities in different address spaces.

* Virtual Proxy: Other information of an object can be cached to delay its access.

* Protection proxy: Check whether the caller's request has necessary permissions.

2. Subject (abstract object interface): defines the same interface for the realsubject object and proxy, so that realsubject can be accessed anywhere Using proxy.

3. realsubject (entity): defines the entity of the proxy.

The following figure shows the order of typical applications:

• Example 1-mathematical proxy

The following example shows the remote proxy of an application to access another application.ProgramObjects in the domain.

The UML class diagram is as follows:

Code

    //  Imath 
    Public     Interface  Imath
{
  Double  Add (  Double  X,  Double  Y );
  Double  Sub (  Double  X,  Double Y );
  Double  Mul (  Double  X,  Double  Y );
  Double  Div (  Double  X,  Double  Y );
}
  //  Entity to implement two interfaces. The marshalbyrefobject interface allows communication between different program domains (processes)  
   Class  Math: marshalbyrefobject, imath
{
  Public     Double  Add (  Double  X,  Double  Y)
{
  Return  X  +  Y;
}
  Public    Double  Sub (  Double  X,  Double  Y)
{
  Return  X  -  Y;
}
  Public     Double  Mul (  Double  X, Double  Y)
{
  Return  X  *  Y;
}
  Public     Double  Div (  Double  X,  Double  Y)
{
  Return  X  / Y;
}
}
  //  Remote proxy object proxy, including an instance of math, which is used to process, add, subtract, multiply, and divide  
    Class  Mathproxy: imath
{
Math math;
  Public  Mathproxy ()
{
Math  =     New  Math ();
}
  Public    Double  Add (  Double  X,  Double  Y)
{
  Return  Math. Add (x, y );
}
  Public     Double  Sub (  Double  X,  Double  Y)
{
 Return  Math. sub (x, y );
}
  Public     Double  Mul (  Double  X,  Double  Y)
{
  Return  Math. Mul (x, y );
}
  Public     Double  Div ( Double  X,  Double  Y)
{
  Return  Math. Div (x, y );
}
}

  //  Customer application testing  
    Class  Client
{
[Stathread]
  Static     Void  Main ( String  [] ARGs)
{
Mathproxy P  =     New  Mathproxy ();
Console. writeline (  "  4 + 2 = {0}  "  , P. Add (  4  ,  2  ));
Console. writeline (  "  4-2 = {0} "  , P. sub (  4  ,  2  ));
Console. writeline (  "  4*2 = {0}  "  , P. Mul (  4  ,  2  ));
Console. writeline (  "  4/2 = {0}  " , P. Div (  4  ,  2  ));
Console. Read ();
}
} 

 

 

• Strengths and weaknesses

When an object is generated on a remote machine through the network, the speed may be slow. In this case, the remote proxy mode can be applied to mask the process of generating the object from the network, and the system speed will be faster; for the loading of large images, the virtual proxy mode allows loading in the background, and the proxy object used in the foreground optimizes the overall running speed; the protect proxy can verify the permission to reference real objects.

The disadvantage of the proxy mode is that the request processing speed slows down and extra work is required to implement the proxy mode.

• Application scenarios

The classification of the proxy mode described above is the scenario where it can be applied.