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Sergio Molina

Creating an object stream from a JDBC ResultSet

The introduction of  features Stream API and Lambda in Java 8 enables us to make an elegant conversion from a JDBC ResultSet to a stream of objects just providing a mapping function. Such function could be, of course, a lambda. Basically, the idea is to generate a Stream using a ResultSet as Supplier:
 
 
 
 
 
 
 

public class ResultSetSupplier implements Supplier<T>{

		private final ResultSet rs;
		private final Function<ResultSet, T> mappingFunction;

		private ResultSetSupplier(ResultSet rs,
                    Function<ResultSet, T> mappingFunction) {
			this.rs = rs;
			this.mappingFunction = mappingFunction;
		}

		@Override
		public T get() {
			try {
				if (rs.next())
					return mappingFunction.apply(rs);
			} catch (SQLException e) {
				e.printStackTrace();
			}
			return null;
		}
	}

Parameter mappingFunction, which might be a lambda expression, is used to build T instances from a ResultSet. Just like ActiveRecord pattern, every row in such ResultSet maps to an instance of T, where columns are attributes of T. Let’s consider class City:

public class City{
		String city;
		String country;
		public City(String city, String country) {
			this.city = city;
			this.country = country;
		}
		public String getCountry() {
			return country;
		}
		@Override
		public String toString() {
			return "City [city=" + city + ", country=" + country + ";]";
		}
		@Override
		public int hashCode() {
			final int prime = 31;
			int result = 1;
			result = prime * result + ((city == null) ? 0 : city.hashCode());
			result = prime * result
					+ ((country == null) ? 0 : country.hashCode());
			return result;
		}
		@Override
		public boolean equals(Object obj) {
			if (this == obj)
				return true;
			if (obj == null)
				return false;
			if (getClass() != obj.getClass())
				return false;
			City other = (City) obj;
			if (city == null) {
				if (other.city != null)
					return false;
			} else if (!city.equals(other.city))
				return false;
			if (country == null) {
				if (other.country != null)
					return false;
			} else if (!country.equals(other.country))
				return false;
			return true;
		}
	}

The mapping function for City objects could be a lambda expression like the following:

(ResultSet rs) -> {
                try
                {
                  return new City(rs.getString("city"), rs.getString("country"));
		} catch (Exception e) {
		  return null;
		}}

We have assumed database columns are called city and country, respectively. Although both PreparedStatement and ResultSet implement AutoCloseable interface, as a  resultSet must be provided to create the object stream, it does make sense to close such resultSet when the stream is closed as well. A possible approach could be to use a proxy to intercept method invocation on the object stream. Thus, as close() method is invoked on the proxy, it will invoke close() on the provided resultSet. All method invocations will be invoked on the object stream as well, in order to be able to provide all Stream features. That is easy to achieve using a proxy. Let’s have a look. We will have a proxy factory and a invocation handler:

public class ResultSetStreamInvocationHandler<T> implements InvocationHandler{

  private Stream<T> stream; // proxy will intercept method calls to such stream
  private PreparedStatement st;
  private ResultSet rs;

  public void setup(PreparedStatement st, Function<ResultSet, T> mappingFunction)
  throws SQLException{
    // PreparedStatement must be already setup in order
    // to just call executeQuery()
    this.st = st;
    rs = st.executeQuery();
    stream = Stream.generate(new ResultSetSupplier(rs, mappingFunction));
  }

  @Override
  public Object invoke(Object proxy, Method method, Object[] args)
  throws Throwable {

    if (method == null)
      throw new RuntimeException("null method null");

    // implement AutoCloseable for PreparedStatement
    // as calling close() more than once has no effects
    if (method.getName().equals("close") && args == null){
    // invoked close(), no arguments
      if (st != null){
        st.close(); // closes ResultSet too
      }
    }

    return method.invoke(stream, args);
  }

private class ResultSetSupplier implements Supplier<T>{

  private final ResultSet rs;
  private final Function<ResultSet, T> mappingFunction;

  private ResultSetSupplier(ResultSet rs, Function<ResultSet, T> mappingFunction) {
    this.rs = rs;
    this.mappingFunction = mappingFunction;
  }

  @Override
  public T get() {
    try {
      if (rs.next())
        return mappingFunction.apply(rs);
    } catch (SQLException e) {
     e.printStackTrace();
    }
    return null;
  }
}

}

Please note how invoke is used to intercept method calls. In case close() is called, close() is called on PreparedStatement as well. For every method called, the corresponding method call is invoked in the stream being proxied. And the factory:

public class ResultSetStream<T>{

	@SuppressWarnings("unchecked")
	public Stream<T> getStream(PreparedStatement st,
            Function<ResultSet, T> mappingFunction) throws SQLException{
		final ResultSetStreamInvocationHandler<T> handler =
                    new ResultSetStreamInvocationHandler<T>();
		handler.setup(st, mappingFunction);
		Stream<T> proxy = (Stream<T>) Proxy.newProxyInstance(getClass().getClassLoader(),
                new Class<?>[] {Stream.class},
                handler);
		return proxy;
	}
}

To put it all together, let’s write a simple test to show usage. Mockito will be used to mock both PreparedStatement and ResultSet to avoid running tests against a real database.

public class ResultSetStreamTest {

	private class City{
		String city;
		String country;
		public City(String city, String country) {
			this.city = city;
			this.country = country;
		}
		public String getCountry() {
			return country;
		}
		@Override
		public String toString() {
			return "City [city=" + city + ", country=" + country + "]";
		}
		@Override
		public int hashCode() {
			final int prime = 31;
			int result = 1;
			result = prime * result + getOuterType().hashCode();
			result = prime * result + ((city == null) ? 0 : city.hashCode());
			result = prime * result
					+ ((country == null) ? 0 : country.hashCode());
			return result;
		}
		@Override
		public boolean equals(Object obj) {
			if (this == obj)
				return true;
			if (obj == null)
				return false;
			if (getClass() != obj.getClass())
				return false;
			City other = (City) obj;
			if (!getOuterType().equals(other.getOuterType()))
				return false;
			if (city == null) {
				if (other.city != null)
					return false;
			} else if (!city.equals(other.city))
				return false;
			if (country == null) {
				if (other.country != null)
					return false;
			} else if (!country.equals(other.country))
				return false;
			return true;
		}
		private ResultSetStreamTest getOuterType() {
			return ResultSetStreamTest.this;
		}
	}

	private String[][] data = new String[][]{
			{"Karachi", "Pakistan"},
			{"Istanbul", "Turkey"},
			{"Hong Kong", "China"},
			{"Saint Petersburg", "Russia"},
			{"Sydney", "Australia"},
			{"Berlin", "Germany"},
			{"Madrid", "Spain"}
		};

	private int timesCalled;
	private PreparedStatement mockPST;
	private ResultSet mockRS;

	@Before
	public void setup() throws SQLException{
		timesCalled = -1;
		mockRS = mock(ResultSet.class);
		mockPST = mock(PreparedStatement.class);

		when(mockRS.next()).thenAnswer(new Answer<Boolean>() {

			@Override
			public Boolean answer(InvocationOnMock invocation) throws Throwable {
				if (timesCalled++ >= data.length)
					return false;
				return true;
			}
		});

		when(mockRS.getString(eq("city"))).thenAnswer(new Answer<String>() {

			@Override
			public String answer(InvocationOnMock invocation) throws Throwable {
				return data[timesCalled][0];
			}
		});
		when(mockRS.getString(eq("country"))).thenAnswer(new Answer<String>() {

			@Override
			public String answer(InvocationOnMock invocation) throws Throwable {
				return data[timesCalled][1];
			}
		});

		when(mockPST.executeQuery()).thenReturn(mockRS);
	}

	@Test
	public void simpleTest() throws SQLException{

		try (Stream<City> testStream = new ResultSetStream<City>().getStream(mockPST,
				(ResultSet rs) -> {try {
					return new City(rs.getString("city"), rs.getString("country"));
				} catch (Exception e) {
					return null;
				}})){

			Iterator<City> cities = testStream.filter(
					city -> !city.getCountry().equalsIgnoreCase("China"))
					.limit(3).iterator();

			assertTrue(cities.hasNext());
			assertEquals(new City("Karachi", "Pakistan"), cities.next());

			assertTrue(cities.hasNext());
			assertEquals(new City("Istanbul", "Turkey"), cities.next());

			assertTrue(cities.hasNext());
			assertEquals(new City("Saint Petersburg", "Russia"), cities.next());

			assertFalse(cities.hasNext());
		}

	}

}
  • Download full source code on Github.
Reference: Creating an object stream from a JDBC ResultSet from our JCG partner Sergio Molina at the TODOdev blog.
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T Murphy
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T Murphy

Sergio, thank you for writing this article. I had a question. Is it possible that the Stream can be infinite? stream = Stream.generate(new ResultSetSupplier(resultSet, mappingFunction)); is .limit the only option to terminate the Stream? Is there any means of closing the Stream from within a foreach block final Stream stream = (new ResultSetStream()).getStream(statement, mappingFunction); stream.forEach(r -> { System.out.format(“Writing row.. %s%n”, r); if(r != null){ write(bufferedWriter, r.toString()); } });

Sergio
Guest

Hi

Thanks for the feedback. There are some interesting comments on that subject at https://tododev.wordpress.com/2014/08/14/creating-and-object-stream-from-a-jdbc-resultset/

BR
Sergio