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  1. DZone
  2. Coding
  3. Java
  4. Java 8: Lambda Expressions vs Auto Closeable

Java 8: Lambda Expressions vs Auto Closeable

By 
Mark Needham user avatar
Mark Needham
·
Mar. 03, 14 · Interview
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If you used earlier versions of Neo4j via its Java API with Java 6 you probably have code similar to the following to ensure write operations happen within a transaction:

public class StylesOfTx
{
    public static void main( String[] args ) throws IOException
    {
        String path = "/tmp/tx-style-test";
        FileUtils.deleteRecursively(new File(path));
 
        GraphDatabaseService db = new GraphDatabaseFactory().newEmbeddedDatabase( path );
 
        Transaction tx = db.beginTx();
        try 
        {
            db.createNode();
            tx.success();
        } 
        finally 
        {
            tx.close();
        }
    }
}

In Neo4j 2.0 Transaction started extending AutoCloseable which meant that you could use ‘try with resources’ and the ‘close’ method would be automatically called when the block finished:

public class StylesOfTx
{
    public static void main( String[] args ) throws IOException
    {
        String path = "/tmp/tx-style-test";
        FileUtils.deleteRecursively(new File(path));
 
        GraphDatabaseService db = new GraphDatabaseFactory().newEmbeddedDatabase( path );
 
        try ( Transaction tx = db.beginTx() )
        {
            Node node = db.createNode();
            tx.success();
        }
    }
}

This works quite well although it’s still possible to have transactions hanging around in an application when people don’t use this syntax – the old style is still permissible.

In Venkat Subramaniam’s Java 8 book he suggests an alternative approach where we use a lambda based approach:

public class StylesOfTx
{
    public static void main( String[] args ) throws IOException
    {
        String path = "/tmp/tx-style-test";
        FileUtils.deleteRecursively(new File(path));
 
        GraphDatabaseService db = new GraphDatabaseFactory().newEmbeddedDatabase( path );
 
        Db.withinTransaction(db, neo4jDb -> {
            Node node = neo4jDb.createNode();
        });
    }
 
    static class Db {
        public static void withinTransaction(GraphDatabaseService db, Consumer<GraphDatabaseService> fn) {
            try ( Transaction tx = db.beginTx() )
            {
                fn.accept(db);
                tx.success();
            }
        }
    }
}

The ‘withinTransaction’ function would actually go on GraphDatabaseService or similar rather than being on that Db class but it was easier to put it on there for this example.

A disadvantage of this style is that you don’t have explicit control over the transaction for handling the failure case – it’s assumed that if ‘tx.success()’ isn’t called then the transaction failed and it’s rolled back. I’m not sure what % of use cases actually need such fine grained control though.

Brian Hurt refers to this as the ‘hole in the middle pattern‘ and I imagine we’ll start seeing more code of this ilk once Java 8 is released and becomes more widely used.

Java (programming language)

Published at DZone with permission of Mark Needham, DZone MVB. See the original article here.

Opinions expressed by DZone contributors are their own.

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  • Introducing Graph Concepts in Java With Eclipse JNoSQL, Part 2: Understanding Neo4j
  • How to Introduce a New API Quickly Using Micronaut
  • Introducing Graph Concepts in Java With Eclipse JNoSQL

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