what may or may not be in java 7 from the java road trip

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Oracle talked about Java 7 at the first stop of the Java Road Trip. The main speaker was Brian Goetz, author of Java Concurrency In Practice.

Despite the disclaimer that everything including syntax is subject to change, the talk was pretty interesting.  Here are my notes.  (this was largely written on my iPad; please excuse any typos)

Modularity

  • The JRE download is 13mb, but most code doesn’t get run by ordinary apps
  • The monolithic jdk makes new releases of platform take longer to roll out
  • Modularizing apps helps with jar hell and finding out at runtime that something is missing/wrong
  • Will provide mechanism for apps to express dependencies in way useful to both humans and tools
  • Based on concepts in Maven and OSGI
  • Current draft has: Module-info.java defining module and version number along with what module and versions or ranges you depend on
  • Classpath will be legacy mode, preferred mode will be to turn your app into a module

Multi lingual support

  • JVM is a managed code environment
  • Scala is a lot like java, but other dynamic languages (like Ruby) run slower because need to rely on reflection or code generation.
  • JVM is more like Smalltalk than like Java , but some places is tied to Java
  • Da Vinci machine project is targeted to adding features to better support dynamic languages. In particular dynamic method linkage which is being able to select version of method based on type of arguments
  • Invokedynamic vs invokevirtual for method selection and linking. Once calls stabilize, don’t look up in vm anymore, then inlines and is as fast as in  java.  This is a goal, need to get there.

Productivity

  • Project coin – add half a dozen small language changes to simplify everyday tasks.  Would be at  level of enhanced for loop
  1. Reduce boilerplate of generics with the the diamond operator (not really an operator) 
    Map<String, String> map  = new HashMap<>();
  2. Better numerical literals to make long numbers more readable.  Similarly for binary (long string of 0’s and 1’s) 
    long cc= 123_5678_567;
  3. Collection literals – declare inline like we do with arrays and hard coded data.  More declarative. Like associative array in Perl, but may not go that far
  4. Automatically close resources in try/catch.  Better idiom 
    try (InputStream in = createInputStream()){
// code that reads in from stream goes here
} // compiler will call close for you here

Performance

  • Goal: facilitate scalability across multiple cores
  • Fork join extend recursive action to split into subtasks and join to get answer.
  • Will be added to concurrency utilities.
  • Don’t have to tell it how many cores you have
  • ParallelIntArray class automates common operations filter, map and reduce so can say what want to do declaratively

Closures

  • Saving the best for last.
  • Like anonymous inner classes but without boilerplate of anonymous inner classes
  • Still debating syntax
  • Still debating whether return type should be declared
  • Still discussing how to extend interfaces (will be used to add closures for Collections).  Options are “static extension methods” like in C# where you statically “pretend there were these methods are on the class and call the static methods instead.”  Also discussing ” virtual extension methods.”  Either way the closures project will finally address the issue of api evolution
#(int x)(x*3).domorestuff()

postgresql and jdbc

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In postgresql – selecting maximum for each group, we saw the actual stored function.  That was the easy part.  Then there was dealing with Postgresql and JDBC.  I encountered a few surprises while doing this.

Calling the stored function

I expected to use JDBC’s CallableStatement since I was calling a stored procedure.  Nope.  The proper way to call it is:


PreparedStatement stmt = conn.prepareStatement("select * from highlighted_topic_per_forum(?) limit ?");

stmt.setString(1, formattedDate);
stmtp.setInt(2, limit);

Passing a timestamp

Originally, I was passing a timestamp into the stored function by calling stmt.setTimestamp() and a type of “timestamp without time zone”.  Try as I may, postgresql didn’t like this.  It complains $1 not found.  I eventually decided to pass the timestamp as a string.

Passing an array

I was originally planning to pass the category ids via JDBC.  Given what what I experienced with timestamps, I wasn’t so enthusiastic about trying this on an application I work on in my free time.  When I saw the advice online to extend the Array class, I decided to hard code the two category ids.

postgresql – selecting maximum for each group

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I had a requirement to return the thread with the most replies in each forum at JavaRanch‘s Coderanch forums.  In Postgresql 8.4, this would be very easy – just use the window functions.  Unfortunately, we aren’t on Postgresql 8.4 yet.  The other common pattern is something like

select stuff
from mytable t1
where date = (select max(date)
from mytable t2
where t1.key= t2.key)

This doesn’t work well for me either because the value I am comparing (number of posts is dynamic.)  I decided to use a stored procedure to “simplify” things.  I’ve written some stored procedures in Postgresql to do updates before and stored procedures in other languages to do queries so this didn’t seem like a huge task.

Postgresql calls them stored functions, so let’s proceed.  First you need to create a type to represent a row that gets returned by the stored function.

CREATE TYPE highlighted_topic_per_forum_holder AS
(last_user_id integer,
post_time timestamp without time zone,
&lt;other columns go here&gt;);

Then you create the stored procedure.  The outer loop goes through each forum.  The inner loop is the SQL that finds the post with the most replies that was posted to in some time period.  It uses a nested query with a limit to return only 1 thread per forum.  The rest of the SQL adds the relevant data.  See postgresql and JDBC for why it takes varchar rather than timestamp.

CREATE or replace FUNCTION highlighted_topic_per_forum(character varying)
RETURNS SETOF highlighted_topic_per_forum_holder AS
$BODY$
DECLARE
forum RECORD;
r highlighted_topic_per_forum_holder%rowtype;
BEGIN
for forum in EXECUTE 'select forum_id from jforum_forums where categories_id in (1,7)' loop
for r in EXECUTE 'select p.user_id AS last_user_id, p.post_time, p.attach AS attach, t.* '
|| 'from jforum_topics t, jforum_posts p, '
|| '(select topic_id, count(*) from jforum_posts '
|| ' where post_time &gt;= date '' ' || $1 || ' '' '
|| ' and forum_id = ' || forum.forum_id
|| ' AND need_moderate = 0 '
|| ' group by topic_id order by 2 desc limit 1) nested '
|| ' where p.topic_id = nested.topic_id '
|| ' and p.post_id = t.topic_last_post_id '
|| ' order by post_time desc' loop
return next r;
end loop;
end loop;
return;
END;
$BODY$
LANGUAGE 'plpgsql';