如何将 PrintWriter 转换为 String 或写入文件？

Question

I am generating dynamic page using JSP, I want to save this dynamically generated complete page in file as archive.

In JSP, everything is written to PrintWriter out = response.getWriter();

At the end of page, before sending response to client I want to save this page, either in file or in buffer as string for later treatment.

How can I save Printwriter content or convert to String ?

Answer 1

To get a string from the output of a PrintWriter , you can pass a StringWriter to a PrintWriter via the constructor:

@Test
public void writerTest(){
    StringWriter out    = new StringWriter();
    PrintWriter  writer = new PrintWriter(out);

    // use writer, e.g.:
    writer.print("ABC");
    writer.print("DEF");

    writer.flush(); // flush is really optional here, as Writer calls the empty StringWriter.flush
    String result = out.toString();

    assertEquals("ABCDEF", result);
}

Answer 2

Why not use StringWriter instead? I think this should be able to provide what you need.

So for example:

StringWriter strOut = new StringWriter();
...
String output = strOut.toString();
System.out.println(output);

Answer 3

It will depend on: how the PrintWriter is constructed and then used.

If the PrintWriter is constructed 1st and then passed to code that writes to it, you could use the Decorator pattern that allows you to create a sub-class of Writer, that takes the PrintWriter as a delegate, and forwards calls to the delegate, but also maintains a copy of the content that you can then archive.

public class DecoratedWriter extends Writer
{
   private final Writer delegate;

   private final StringWriter archive = new StringWriter();

   //pass in the original PrintWriter here
   public DecoratedWriter( Writer delegate )
   {
      this.delegate = delegate;
   }

   public String getForArchive()
   { 
      return this.archive.toString();
   } 

   public void write( char[] cbuf, int off, int len ) throws IOException
   {
      this.delegate.write( cbuf, off, len );
      this.archive.write( cbuf, off, len );
   }

   public void flush() throws IOException
   {
      this.delegate.flush();
      this.archive.flush();

   } 

   public void close() throws IOException
   {
      this.delegate.close();
      this.archive.close();
   }
}

Answer 4

You cannot get it with just your PrintWriter object. It flushes the data, and does not hold any content within itself. This isn't the object you should be looking at to get the entire string,

Answer 5

Along similar lines to what cdc is doing - you can extend PrintWriter and then create and pass around an instance of this new class.

Call getArchive() to get a copy of the data that's passed through the writer.

public class ArchiveWriter extends PrintWriter {
    private StringBuilder data = new StringBuilder();

    public ArchiveWriter(Writer out) {
        super(out);
    }

    public ArchiveWriter(Writer out, boolean autoFlush) {
        super(out, autoFlush);
    }

    public ArchiveWriter(OutputStream out) {
        super(out);
    }

    public ArchiveWriter(OutputStream out, boolean autoFlush) {
        super(out, autoFlush);
    }

    public ArchiveWriter(String fileName) throws FileNotFoundException {
        super(fileName);
    }

    public ArchiveWriter(String fileName, String csn) throws FileNotFoundException, UnsupportedEncodingException {
        super(fileName, csn);
    }

    public ArchiveWriter(File file) throws FileNotFoundException {
        super(file);
    }

    public ArchiveWriter(File file, String csn) throws FileNotFoundException, UnsupportedEncodingException {
        super(file, csn);
    }

    @Override
    public void write(char[] cbuf, int off, int len) {
        super.write(cbuf, off,len);
        data.append(cbuf, off, len);
    }

    @Override
    public void write(String s, int off, int len) {
        super.write(s, off,len);
        data.append(s, off, len);
    }

    public String getArchive() {
        return data.toString();
    }
}

Answer 6

我认为最好的方法是在 StringBuffer 等其他对象中准备您的响应，并将其内容推送到响应中，然后将存储在该变量中的内容保存到文件中。

Answer 7

This helped me: for obtaining a SOAP-able object as XML string.

JAXBContext jc = JAXBContext.newInstance(o.getClass());
Marshaller m = jc.createMarshaller();
StringWriter writer = new StringWriter();
m.marshal( o, new PrintWriter(writer) );
return writer.toString();