When to use @Singleton in a Jersey resource

Question

I have a Jersey resource that access the database. Basically it opens a database connection in the initialization of the resource. Performs queries on the resource\'s methods.

Answer 1

Rather than thinking about making the resource a singleton, focus more on managing backend, service type objects like your ResponseGenerator class as singletons, which obviously shouldn't be instantiated every request.

Making the resource a singleton as well is one way of managing ResponseGenerator as a singleton, but it's not the only or necessarily the best way, see Access external objects in Jersey Resource class and How to wire in a collaborator into a Jersey resource? for ways to inject this into non-singleton resources.

Note that your ResponseGenerator class would need work before it would function as a singleton, whether injected into a per-request resource or instantiated in a singleton resource. It's not thread safe, and you would open a single connection on startup and reuse it across requests, which won't work, you should use a connection pool to do the heavy lifting of efficiently + safely reusing connections across requests.

Some comments on best practices for the code will also be helpful.

You'll get better responses on http://codereview.stackexchange.com, but:

• ResponseGenerator is a poor name for a class (just about everything in a web application is a response generator).

• don't use String as the return type of your service and object, use proper typed objects (eg it sounds like you're returning a java.util.List of something).

• Don't swallow your SQLException, bubble it up to allow Jersey to generate a 5xx series response code in your resource.

• Use final member variables.

• Your log object should be static.

Answer 2

You best option is to use a framework like Spring with Jersey which I outlined in a similar post. The only difference is that instead of injecting a service bean you would inject a pooled DataSource and this can easily be configured using c3p0.

Example applicationContext.xml, notice the "scope" is set to prototype which is equivalent to a singleton in Spring parlance.

<bean id="pooledDataSource" scope="prototype" class="com.mchange.v2.c3p0.ComboPooledDataSource" destroy-method="close">
    <property name="jdbcUrl" value="${jpa.url}" />
    <property name="user" value="${jpa.username}" />
    <property name="password" value="${jpa.password}" />
    <property name="initialPoolSize" value="1" />
    <property name="minPoolSize" value="1" />
    <property name="maxPoolSize" value="3" />
    <property name="idleConnectionTestPeriod" value="500" />
    <property name="acquireIncrement" value="1" />
    <property name="maxStatements" value="50" />
    <property name="numHelperThreads" value="1" />
</bean>

In your MyResource.java you would simply add the following and Spring would inject it appropriately.

private DataSource pooledDataSource;
public void setPooledDataSource(DataSource pooledDataSource) {
    this.pooledDataSource = pooledDataSource;
}

Then you could change your ResponseGenerator to accept the DataSource and use this to query the database.