异常模板代码

看这篇文章：http://tutorials.jenkov.com/java-exception-handling/exception-handling-templates.html

再录一下：

一个异常捕获后，在 finally 里中再捕获异常，抛出异常会覆盖先前的异常信息，所以需要清晰的判断每个可能的异常，如此不会遗漏被覆盖的异常。如果代码复杂，异常判断是一键冗余的事情，所以会用模板的方式来简化工作，减少出错。

其实就是讲一个共通的代码如何更加优雅的公用。

 Input       input            = null;
    IOException processException = null;
    try{
        input = new FileInputStream(fileName);

        //...process input stream...
    } catch (IOException e) {
        processException = e;
    } finally {
       if(input != null){
          try {
             input.close();
          } catch(IOException e){
             if(processException != null){
                throw new MyException(processException, e,
                  "Error message..." +
                  fileName);
             } else {
                throw new MyException(e,
                    "Error closing InputStream for file " +
                    fileName;
             }
          }
       }
       if(processException != null){
          throw new MyException(processException,
            "Error processing InputStream for file " +
                fileName;
    }

模板模式：

public abstract class InputStreamProcessingTemplate {

    public void process(String fileName){
        IOException processException = null;
        InputStream input = null;
        try{
            input = new FileInputStream(fileName);

            doProcess(input);
        } catch (IOException e) {
            processException = e;
        } finally {
           if(input != null){
              try {
                 input.close();
              } catch(IOException e){
                 if(processException != null){
                    throw new MyException(processException, e,
                      "Error message..." +
                      fileName);
                 } else {
                    throw new MyException(e,
                        "Error closing InputStream for file " +
                        fileName;
                 }
              }
           }
           if(processException != null){
              throw new MyException(processException,
                "Error processing InputStream for file " +
                    fileName;
        }
    }

    //override this method in a subclass, to process the stream.
    public abstract void doProcess(InputStream input) throws IOException;
}

调用：

  new InputStreamProcessingTemplate(){
        public void doProcess(InputStream input) throws IOException{
            int inChar = input.read();
            while(inChar !- -1){
                //do something with the chars...
            }
        }
    }.process("someFile.txt");

接口实现的办法：

public interface InputStreamProcessor {
    public void process(InputStream input) throws IOException;
}


public class InputStreamProcessingTemplate {

    public void process(String fileName, InputStreamProcessor processor){
        IOException processException = null;
        InputStream input = null;
        try{
            input = new FileInputStream(fileName);

            processor.process(input);
        } catch (IOException e) {
            processException = e;
        } finally {
           if(input != null){
              try {
                 input.close();
              } catch(IOException e){
                 if(processException != null){
                    throw new MyException(processException, e,
                      "Error message..." +
                      fileName;
                 } else {
                    throw new MyException(e,
                        "Error closing InputStream for file " +
                        fileName);
                 }
              }
           }
           if(processException != null){
              throw new MyException(processException,
                "Error processing InputStream for file " +
                    fileName;
        }
    }
}

调用：

new InputStreamProcessingTemplate()
        .process("someFile.txt", new InputStreamProcessor(){
            public void process(InputStream input) throws IOException{
                int inChar = input.read();
                while(inChar !- -1){
                    //do something with the chars...
                }
            }
        });

静态模板：

public class InputStreamProcessingTemplate {

    public static void process(String fileName,
    InputStreamProcessor processor){
        IOException processException = null;
        InputStream input = null;
        try{
            input = new FileInputStream(fileName);

            processor.process(input);
        } catch (IOException e) {
            processException = e;
        } finally {
           if(input != null){
              try {
                 input.close();
              } catch(IOException e){
                 if(processException != null){
                    throw new MyException(processException, e,
                      "Error message..." +
                      fileName);
                 } else {
                    throw new MyException(e,
                        "Error closing InputStream for file " +
                        fileName;
                 }
              }
           }
           if(processException != null){
              throw new MyException(processException,
                "Error processing InputStream for file " +
                    fileName;
        }
    }
}

调用：

 InputStreamProcessingTemplate.process("someFile.txt",
        new InputStreamProcessor(){
            public void process(InputStream input) throws IOException{
                int inChar = input.read();
                while(inChar !- -1){
                    //do something with the chars...
                }
            }
        });