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From "Luke Lu (JIRA)" <>
Subject [jira] Created: (SOLR-388) Refactor ResponseWriters and Friends.
Date Sun, 21 Oct 2007 01:03:50 GMT
Refactor ResponseWriters and Friends.

                 Key: SOLR-388
             Project: Solr
          Issue Type: Improvement
          Components: clients - java
    Affects Versions: 1.2, 1.3
            Reporter: Luke Lu

When developing custom request handlers, it's often necessary to create corresponding response
writers that extends existing ones. In our case, we want to augment the result list (more
attributes other than numFound, maxScore, on the fly per doc attributes that are not fields
etc.) , only to find JSONWriter and friends are private to the package. We could copy the
whole thing and modify it, but it wouldn't take advantage of recent fixes like Yonik's FastWriter
changes without tedious manual intervention. I hope that we can can *at least* extends it
and overrides writeVal() to add a new result type to call writeMyType. 

Ideally the ResponseWriter hierarchy could be rewritten to take advantage of a double dispatching
trick to get rid of the ugly if something is instance of someclass else ... list, as it clearly
doesn't scale well with number of types (n) and depth (d) of the writer hierarchy, as the
complexity would be O(nd), which is obviously inferior to the O(1) double dispatching mechanism.
Some pseudo code here:

// a list of overloaded write method
public void write(SomeType t) {
  // implementation

// an interface for object that support the scheme
public interface ResponseWritable {
  public abstract void write(ResponseWriter writer);

// Sometype needs to implement the ResponseWritable interface
// to facilitate double dispatching
public void write(ResponseWriter writer) {

So when adding a new MyType and MySomeResponseWriter, we only need to add these two files
without having to muck with the writeVal if-then-else list. Note, you still need to use the
if else list for builtin types and any types that you can't modify in the write(Object) method.

// implements the ResponseWritable interface
public write(ResponseWriter writer) {

//  only need to implement this method
public void write(MyType t) {
  // implementation

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