Java 8 goodie: SQL ResultSet Streams

This post was originally published over at jooq.org as part of a special series focusing on all things Java 8, including how take advantage of lambda expressions, extension methods, and other great stuff. You’ll find the source code on GitHub.

Checked Exceptions

Yes. Unfortunately, those beasts from the past still haunt us, more than ever when we’re using Java 8′s lambda expressions. Already before Java 8′s release, there are a couple of Stack Overflow questions related to the subject.

• Mandatory checked exceptions handling in lambda expressions for standard Java 8 functional interfaces
• Lambda function that throws exception?
• Lambda-Streams, Filter by Method with Exception

Let’s remember how the IOExceptions caused issues when traversing the file system. Unless you write your own utility, you’ll have to resort to this beauty:

Arrays.stream(dir.listFiles()).forEach(file -> {
    try {
        System.out.println(file.getCanonicalPath());
    }
    catch (IOException e) {
        throw new RuntimeException(e);
    }
 
    // Ouch, my fingers hurt! All this typing!
});

We think it is safe to say:

Java 8 and checked exceptions don’t match.

A workaround is to write your own CheckedConsumer that wraps the checked exception. Such a consumer will be highly reusable, but… Did you think of all the other FunctionalInterfaces? There are quite a few of them in the java.util.function package:

Some of the many types in java.util.function

jOOλ – Fixing lambda in Java 8 

While writing this Java 8 blog series, we’ve constantly run into the need to wrap checked exceptions inside lambda expressions. And what do we geeks do when we frequently run into a problem? We fix it! And we have created jOOλ (also jOOL, jOO-Lambda), ASL 2.0 licensed, where we have duplicated pretty much every FunctionalInterface that is available from the JDK to support checked exceptions. Here’s how you would use jOOλ in the above example:

Arrays.stream(dir.listFiles()).forEach(
    Unchecked.consumer(file -> {
        // Throw all sorts of checked exceptions
        // here, we don't care...
        System.out.println(file.getCanonicalPath());
    })
);

The above example shows how you can simply ignore and pass checked exceptions as RuntimeExceptions. If you actually want to handle them, you can pass an exception handler lambda:

Arrays.stream(dir.listFiles())
      .forEach(Unchecked.consumer(
 
    file -> {
        System.out.println(file.getCanonicalPath());
    },
    e -> {
        log.info("Log stuff here", e);
        throw new MyRuntimeException(e);
    }
);

The second example now seems equally verbose, but don’t worry. You will probably reuse that exception handler and fall back to this:

Arrays.stream(dir.listFiles())
      .forEach(Unchecked.consumer(
    file -> {
        System.out.println(file.getCanonicalPath());
    },
    myExceptionHandler
);

jOOλ – Providing JDBC ResultSet Streams

Unfortunately, most efforts in the Java 8 Streams API were made in the area of correctly implementing parallelisable streams. While this is very useful for those of us actually doing parallel computing, for most others better integration with legacy APIs would have been better. One API that seriously deserves some lifting is JDBC, and we’ve blogged about this before. With jOOλ, you can now generate Streams directly fromResultSets or even from PreparedStatements. Here’s how you prepare:

Class.forName("org.h2.Driver");
try (Connection c = getConnection()) {
    String sql = "select schema_name, is_default " +
                 "from information_schema.schemata " +
                 "order by schema_name";
 
    try (PreparedStatement stmt = c.prepareStatement(sql)) {
        // code here
    }
}

Now, all you have to do when using jOOλ is stream your PreparedStatements as such:

SQL.stream(stmt, Unchecked.function(rs ->
    new SQLGoodies.Schema(
        rs.getString("SCHEMA_NAME"),
        rs.getBoolean("IS_DEFAULT")
    )
))
.forEach(System.out::println);

Where SQLGoodies.Schema is just an ordinary POJO. Some of thestream() method’s signatures are these ones:

public static <T> Stream<T> stream(
    PreparedStatement stmt,
    Function<ResultSet, T> rowFunction
);
 
public static <T> Stream<T> stream(
    PreparedStatement stmt,
    Function<ResultSet, T> rowFunction,
    Consumer<? super SQLException> exceptionHandler
);

Others are available as well.

That is awesome, isn’t it?

JDBC ResultSets should be Java 8 Streams.

Too bad, the above code didn’t make it into the JDK 8, as this would have been a chance to finally greatly improve on the JDBC API. Another, similar attempt at improving things has been done here by Julian Exenberger.

Java 8 alternatives of writing SQL

We’ve also published a couple of alternatives to jOOλ, using Java 8 with SQL here:http://www.jooq.org/java-8-and-sql

Conclusion

While Java 8′s lambda expressions are awesome, the new Streams API is pretty incomplete. When implementing the above, we had to implement our own ResultSetIterator, and write all this mess to wrap the iterator in aStream:

StreamSupport.stream(
    Spliterators.spliteratorUnknownSize(
        new ResultSetIterator<>(
            supplier,
            rowFunction,
            exceptionTranslator
        ), 0
    ), false
);

And it shouldn’t be necessary to write an Iterator in the first place, if only we were able to generate finite streams:

// Unfortunately, this method doesn't exist
Stream.generate(
    // Supplier, generating new POJOs
    () -> {
        rs.next();
        return new SQLGoodies.Schema(
            rs.getString("SCHEMA_NAME"),
            rs.getBoolean("IS_DEFAULT")
        );
    },
 
    // Predicate, terminating the Stream
    () -> { !rs.isLast(); }
);

While jOOλ is an acceptable intermediate solution, and the Guava guys are probably already working out how to fix their library, it is really too bad, that Java 8 is lacking such utility functionality.