Java Conditional Compile - Java条件编译

The conditional compilation practice is used to optionally remove chunks of code from the compiled version of a class. It uses the fact that compilers will ignore any unreachable branches of code. 

To implement conditional compilation, 

* define a static final boolean value as a non-private member of some class 
* place code which is to be conditionally compiled in an if block which evaluates the boolean 
* set the value of the boolean to false to cause the compiler to ignore the branch; otherwise, keep its value as true 

This practice is used mostly for implementing debugging statements, logging statements, and for assertions. With the advent of assert and the Logging API in JDK 1.4, the utility of conditional compilation is probably reduced. 

Example 

public   final   class  Debug {
   // set to false to allow compiler to identify and eliminate
   // unreachable code
   public   static   final   boolean  ON  =   true ;
} 



import  javax.servlet. * ;
import  javax.servlet.http. * ;
import  java.io. * ;

/**
* Uses conditional compilation to optionally print debugging statements.
*/ public   final   class  GenericServlet  extends  HttpServlet {

   public   void  init(ServletConfig aConfig)  throws  ServletException {
     super .init(aConfig);

     // print out a message, but only if Debug.ON evaluates to true
     // if Debug.ON evaluates to false, this entire block of code will be
     // passed over by the compiler, and will not be present in output .class file
     if  (Debug.ON) {
      System.out.println( " **** INIT OF My Controller ***** " );
    }
  }

   public   void  doGet(HttpServletRequest aRequest, HttpServletResponse aResponse)
                                               throws  ServletException, IOException {
     if  (Debug.ON) {
      System.out.println( " Processing doGet " );
    }
    doHttpRequest(aRequest, aResponse);
  }

   public   void  doPost(HttpServletRequest aRequest, HttpServletResponse aResponse)
                                               throws  ServletException, IOException {
     if  (Debug.ON) {
      System.out.println( " Processing doPost " );
    }
    doHttpRequest(aRequest, aResponse);
  }

   // / PRIVATE  //// /

   private   void  doHttpRequest(HttpServletRequest aRequest, HttpServletResponse aResponse)
                                               throws  ServletException, IOException {
      // empty
  }

}