visitor访问者模式-创新互联

1 public interface Resource {
2   void accept(Visitor visitor);
3 }

定义visitor，支持2种类型，reader类资源和writer资源，访问writer资源需要额外一个line参数：

1 public interface Visitor {
2   void visit(Reader reader);
3 
4   void visit(Writer writer, String line);
5 }

1 public interface Reader {
2     String read();
3 }

 1 public class ReaderImpl implements Reader, Resource {
 2     @Override
 3   public String read() {
 4 return "Doc#1 factory riverwood staff manual.";
 5     }
 6 
 7   public void accept(Visitor visitor) {
 8         visitor.visit(this);
 9     }
10 }

 1 public class WriterImpl implements Writer, Resource {
 2 
 3   private String line;
 4   public WriterImpl(String line) {
 5 this.line = line;
 6     }
 7 
 8     @Override
 9   public int write(String line) {
10 return null == line ? 0 : line.length();
11     }
12 
13     @Override
14   public void accept(Visitor visitor) {
15         visitor.visit(this, line);
16     }
17 }

定义实际访问Resource的两个visitor角色：

 1 public class VipVisitorImpl implements Visitor {
 2 
 3     @Override
 4   public void visit(Reader reader) {
 5         String read = reader.read();
 6         System.out.println(read);
 7     }
 8 
 9     @Override
10   public void visit(Writer writer, String line) {
11 int write = writer.write(line);
12 if (write > 0) {
13             System.out.println("write success!");
14         }
15     }
16 }

 1 public class OrdinaryVisitorImpl implements Visitor {
 2 
 3     @Override
 4   public void visit(Reader reader) {
 5         String read = reader.read();
 6         System.out.println(read);
 7     }
 8 
 9     @Override
10   public void visit(Writer writer, String line) {
11         System.out.println("no access to writer!");
12     }
13 
14 }

调用：

 1 String line = "save this line";
 2         Resource reader = new ReaderImpl();
 3         Resource writer = new WriterImpl(line);
 4 
 5         Visitor ordinaryVisitor = new OrdinaryVisitorImpl();
 6         Visitor vipVisitor = new VipVisitorImpl();
 7 
 8         reader.accept(vipVisitor);
 9         reader.accept(ordinaryVisitor);
10 
11         writer.accept(vipVisitor);
12         writer.accept(ordinaryVisitor);

执行输出：

Doc#1 factory riverwood staff manual.

Doc#1 factory riverwood staff manual.

write success!

no access to writer!