Question

I have a MongoService class as

public class MongoService {

    private final Mongo mongo;
    private final String database;
    private static final Logger LOGGER = LoggerFactory.getLogger(MongoService.class);

    public MongoService(@Nonnull final String host, final int port, @Nonnull final String db) throws UnknownHostException {
        mongo = new Mongo(host, port);
        database = db;
    }

    public void putDocument(@Nonnull final DBObject document) {
        LOGGER.info("inserting document - " + document.toString());
        mongo.getDB(database).getCollection(getCollectionName(document)).insert(document, WriteConcern.SAFE);
    }

    public void putDocuments(@Nonnull final List<DBObject> documents) {
        for (final DBObject document : documents) {
            putDocument(document);
        }
    }

}

I want to inject the value of host, port, db into constructor from a external properties file
/storage/local.properties

### === MongoDB interaction === ###
host=127.0.0.1
port=27017
database=contract  

My Spring wireup file looks as follows
wireup.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:util="http://www.springframework.org/schema/util"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util-2.0.xsd">

    <util:properties id="mongoProperties" location="file:///storage//local.properties" />

    <bean id="mongoService" class="com.business.persist.MongoService">
        // TODO
    </bean>
</beans>

Question

How can I pass the value of host, port, db from local.properties file and pass it to the following constructor?

public MongoService(@Nonnull final String host, final int port, @Nonnull final String db) throws UnknownHostException {
        mongo = new Mongo(host, port);
        database = db;
    }

Answer 1

Instead of importing your properties file using the util:properties tag, you want to import it using the context:property-placeholder. The util version simply imports the file as a Properties object, rather than exposing the property values to your configuration. So your setup would be something like:

<context:property-placeholder location="file:///storage//local.properties"/>

Then when you are wiring up your MongoService, you can use the property names in your constructor config, such as

<bean id="mongoService" class="com.business.persist.MongoService">
    <constructor-arg value="${host}"/>
    <constructor-arg value="${port}"/>
    <constructor-arg value="${database}"/>
</bean>

See the spring docs for more details.
On a side note I would consider giving a more descriptive name to each of the properties to avoid collision with other properties that might be defined in your application.