Benji's Blog » Java

Typesafe Hibernate POJO Queries without code generation

benji — Thu, 19 May 2011 23:08:57 +0000

Hibernate is great, but often one has to specify queries as HQL in Strings, or as criteria which allow building of invalid queries.

It would be great to use the java type system to help enforce correct queries. I am aware of some tools to do this, but all the ones I have seen require either a code generation step, or preclude the use of standard getters/setters.

Here is a proof of concept of one approach that I think could work. It allows things like:

    public List<Person> getPeopleByName(final String firstName, final String lastName) {
        return new Query<Person>() {
            public CompletedQuery spec(Person person) { // <- Only Person can be passed to here.
                return where(person.getFirstname())
                    .equalTo(firstName) // <- Only comparisons are valid at this step, a .and(... would be a compilefailure.
                    .and(person.getLastname())
                    .equalTo(lastName)
                    .select(); // <- Omitting this would be a compile failure due to return type.
            }
        }.list(HibernateUtil.getSessionFactory().getCurrentSession()); // <- Method returns a List
    }

A toString on the above Query when passed firstname benji and lastname weber would yield:

FROM Person WHERE firstname = 'benji' AND lastname = 'weber'

If we were to try to supply a non-string value for first name we would get a compile failure.

    public List<Person> getPeopleByName(final String firstName, final String lastName) {
        return new Query<Person>() {
            public CompletedQuery spec(Person person) {
                return where(person.getFirstname())
                    .equalTo(firstName)
                    .and(person.getLastname())
                    .equalTo(5) // <- Compile failure
                    .select();
            }
        }.list(HibernateUtil.getSessionFactory().getCurrentSession());
    }

This approach can even work querying through relationships

    public List<Person> getPeopleByAddress(final Address address) {
        return new Query<Person>() {
            public CompletedQuery spec(Person person) {
                return where(person.getAddress().getId())
                    .equalTo(address.getId()) // < - Only Integers are allowed here.
                    .select();
            }
        }.list(HibernateUtil.getSessionFactory().getCurrentSession());
    }

A toString on the above Query when passed an Address with id 2 would yield:

FROM Person WHERE address.id = 2

To implement this I used a few tricks:

Query is an abstract class that uses Gafter’s Gadget to obtain a Class this is used to get the entity name that we are querying, and later to create an instance.
A Dynamic Proxy class is created for this Class and passed to the spec(T value) method.
This dynamic proxy records all calls made to it. If the call is to a method that returns another mockable object (e.g. getAddress() in the above example) then another recording-proxy is returned.
The query builder then uses the recorded method calls to determine which properties are being queried
The need to return a CompletedQuery interface forces the implementor to type a complete query or they will get a compile failure.
Interfaces represent the valid progressions at each step of the query building. e.g. after a where() you need to supply a comparison equalTo(), or like() etc. After a comparison you can complete the query or add another restriction with and();

Here is the example implementation of Query that enables this. Full demo code here

package uk.co.benjiweber.typesafehibernate.typesafe;
 
import java.lang.reflect.Method;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.List;
import net.sf.cglib.proxy.Enhancer;
import net.sf.cglib.proxy.MethodInterceptor;
import net.sf.cglib.proxy.MethodProxy;
import org.hibernate.Session;
import org.springframework.util.StringUtils;
 
public abstract class Query<T> {
 
    private final Type type;
    private final Recorder<T> recorder;
    private final StringBuilder queryBuilder = new StringBuilder();
 
    public Query() {
        Type superclass = getClass().getGenericSuperclass();
        this.type = ((ParameterizedType) superclass).getActualTypeArguments()[0];
        this.queryBuilder.append("FROM " + getParamClass().getSimpleName());
        recorder = createMockInstance();
        spec(recorder.getObject());
    }
 
    public abstract CompletedQuery spec(T entity);
 
    public interface CompletedQuery {
    }
 
    public <U> Comparison<U> where(U ignore) {
        queryBuilder.append(" WHERE ");
        return comparison(ignore);
    }
 
    private <U> Comparison<U> comparison(U ignore) {
        return new Comparison<U>()  {
 
            public QueryOptions equalTo(U value) {
                return comparison(value, "=");
            }
 
            public QueryOptions notEqualTo(U value) {
                return comparison(value, "!=");
            }
 
            public QueryOptions like(U value) {
                return comparison(value, "LIKE");
            }
 
            private QueryOptions comparison(U value, String operator) {
                boolean quote = value != null && String.class.equals(value.getClass());
 
                queryBuilder.append(recorder.getCurrentPropertyName()).append(" ").append(operator).append(" ").append(quote ? "'" : "").append(value).append(quote ? "'" : "");
                return new AbstractQueryOptions()  {
 
                    public <V> Comparison<V> and(V o) {
                        return Query.this.and(o);
                    }
                };
            }
        };
    }
 
    private <U> Comparison<U> and(U ignore) {
        queryBuilder.append(" AND ");
        return comparison(ignore);
    }
 
    public interface Comparison<U> {
 
        QueryOptions equalTo(U value);
 
        QueryOptions notEqualTo(U value);
 
        QueryOptions like(U value);
    }
 
    abstract static class AbstractQueryOptions implements QueryOptions {
 
        public CompletedQuery select() {
            return new CompletedQuery()  {
            };
        }
    }
 
    public interface QueryOptions {
 
        <V> Comparison<V> and(V o);
 
        CompletedQuery select();
    }
 
    private Recorder<T> createMockInstance() {
        Class<T> cls = getParamClass();
 
        return RecordingObject.create(cls);
    }
 
    private Class<T> getParamClass() {
        return type instanceof Class
                ? (Class<T>) type
                : (Class<T>) ((ParameterizedType) type).getRawType();
 
    }
 
    @Override
    public String toString() {
        return queryBuilder.toString();
    }
 
    public List<T> list(Session session) {
        return session.createQuery(toString()).list();
    }
}
 
class Recorder<T> {
 
    private T t;
    private RecordingObject recorder;
 
    public Recorder(T t, RecordingObject recorder) {
        this.t = t;
        this.recorder = recorder;
    }
 
    public String getCurrentPropertyName() {
        return recorder.getCurrentPropertyName();
    }
 
    public T getObject() {
        return t;
    }
}
 
class RecordingObject implements MethodInterceptor {
 
    private String currentPropertyName = "";
    private Recorder currentMock = null;
 
    @SuppressWarnings("unchecked")
    public static <T> Recorder<T> create(Class<T> cls) {
        Enhancer enhancer = new Enhancer();
        enhancer.setSuperclass(cls);
        final RecordingObject recordingObject = new RecordingObject();
 
        enhancer.setCallback(recordingObject);
        return new Recorder((T) enhancer.create(), recordingObject);
    }
 
    public Object intercept(Object o, Method method, Object[] os, MethodProxy mp) throws Throwable {
        if (method.getName().equals("getCurrentPropertyName")) {
            return getCurrentPropertyName();
        }
        currentPropertyName = StringUtils.uncapitalize(method.getName().replaceAll("^get", ""));
        try {
            currentMock = create(method.getReturnType()); //method.invoke(o, os);
            return currentMock.getObject();
        } catch (IllegalArgumentException e) {
            //non-mockable
            return null;
        }
    }
 
    public String getCurrentPropertyName() {
        return currentPropertyName + (currentMock == null ? "" : ("." + currentMock.getCurrentPropertyName()));
    }
}

Java Abuse: Inline instanceof

benji — Thu, 16 Sep 2010 20:23:40 +0000

One annoyance in Java is having to do instanceof checks on multiple lines. e.g.

if (object instanceof Foo) {
    Foo foo = (Foo)object;
    foo.foo();
}

While this is often a sign of a design failure, there are times when instanceof checks are required often due to framework constraints etc. The above is quite ugly and involves using 3 lines instead of 1 for a single method call. If foo() is a void method we can’t even use the ternary ? : operator to make it more concise.

In c# with Lambdas and Extension methods one can create something like :

    object.When<Foo>(foo => foo.Foo());

Where “When” is an extension method added to all objects which takes a type parameter and a lambda to execute if the type parameter matches. (Implementation available if anyone cares).

Since Java doesn’t have Extension methods or Lambdas yet (Oracle minus minus) we can’t do this. We can however always use reflection and get something like:

package scratchpad;
 
import java.lang.reflect.Method;
import net.sf.cglib.proxy.Enhancer;
import net.sf.cglib.proxy.MethodInterceptor;
import net.sf.cglib.proxy.MethodProxy;
 
import static scratchpad.When.when;
 
public class Let {
	public static void main(String... args) {
		Object a = new Foo();
		Object b = "hello world";
 
		when(Foo.class).isTypeOf(a).foo(); // Prints "foo"
		when(Foo.class).isTypeOf(b).foo(); // Does nothing at all
	}
}
 
class Foo {
	public void foo() {
		System.out.println("foo");
	}
}
 
class When {
	public static <T> Is<T> when(final Class<T> cls) {
		return new Is<T>() {
			@SuppressWarnings("unchecked")
			public T isTypeOf(Object o) {
				return o != null && cls.isAssignableFrom(o.getClass()) 
					? (T)o
					: NullObject.create(cls);
			}
 
		};
	}
 
	public interface Is<T> {
		public T isTypeOf(Object o);
	}
}
 
class NullObject implements MethodInterceptor  {
 
	@SuppressWarnings("unchecked")
	public static <T> T create(Class<T> cls){
		Enhancer enhancer = new Enhancer();
		enhancer.setSuperclass(cls);
		enhancer.setCallback(new NullObject());
		return (T)enhancer.create();
	}
 
	public Object intercept(Object o, Method method, Object[] os, MethodProxy mp) throws Throwable {
		return null;
	}
}

Java Abuse – Currency Pattern

benji — Thu, 18 Jun 2009 17:26:12 +0000

Here’s a sillier one from last night…

People often complain about not being able to return multiple values from a method in Java. I can’t see a good reason for wanting to do this, but some do.

The example I was given was wanting to do:

int foo, bar, baz; list(foo, bar, bar) = func(ponies);

Where func(ponies) returns 3 values which are then assigned to the local variables foo, bar, and baz.

There are plenty of sensible ways of achieving this, but what if we’re trying to get as close as possible to the above method? We can only return one thing from a method, but that could be a chain. We also can’t modify the local foo,bar,baz from inside the list method, but we could if we wrap them in references.

If we abuse the fact that currency characters are valid identifiers in Java, for the ultimate in unreadability; We can use $(x,y) to be a pair of x and y, $$ to terminate a list, and £(x) to either reference or dereference x.

Then we can do something like this ¬_¬

/** OUTPUT
Foo = 1
Bar = 2
Bingo = Badgers
Baz = 3
**/
 
package scratchpad;
 
import static scratchpad.$.$;
import static scratchpad.$$.$$;
import static scratchpad.$.expand;
import static scratchpad.£.£;
 
public class MultipleReturn
{
	public static void main(String... args)
	{
		new MultipleReturn().demo();
	}
 
	public void demo()
	{
		£<Integer> foo = £(-1), bar = £(-1), baz = £(-1);
		£<String> bingo = £("");
 
		expand(foo()).into(foo,bar,bingo,baz);
 
		System.out.println("Foo = " + £(foo));
		System.out.println("Bar = " + £(bar));
		System.out.println("Bingo = " + £(bingo));
		System.out.println("Baz = " + £(baz));
	}
 
	public $<Integer,$<Integer,$<String,$<Integer,$$>>>> foo()
	{
		return $(1,$(2,$("Badgers",$(3,$$))));
	}
}
 
interface Tuple {}
 
class $$ implements Tuple
{
	private $$() { }
	public static $$ $$ = new $$();
}
 
class $<T,U extends Tuple> implements Tuple
{
	public T _1;
	public U _2;
 
	protected $(T t,U u)
	{
		this._1 = t;
		this._2 = u;
	}
 
	public static <T> $<T,$$> $(T t)
	{
		return new $<T,$$>(t,$$);
	}
 
	public static <T,U extends Tuple> $<T,U> $(T t, U u)
	{
		return new $<T,U>(t,u);
	}
 
	public interface Expander
	{
		public void into(£... refs);
	}
 
	public static Expander expand(final $,? extends Tuple> vals)
	{
		return new Expander()
		{
			public void into(scratchpad.£... refs)
			{
				if (refs.length < 1)
					return;
 
				$ current = vals;
				refs[0]._1 = current._1;
				if (current._2 instanceof $$)
					return;
				int i = 1;
				while (!(current._2 instanceof $$))
				{
					if (i >= refs.length)
						return;
					current = ($)current._2;
					refs[i]._1 = current._1;
					i++;
				}
			}
 
		};
 
	}
}
 
class £<T> extends $<T,$$>
{
	protected £(T t)
	{
		super(t,$$);
	}
 
	public static <T> £<T> £(T t)
	{
		return new £<T>(t);
	}
 
	public static <T> T £(£<T> t)
	{
		return t._1;
	}
}

Java Abuse – Ternary Try/Catch

benji — Sun, 14 Jun 2009 13:03:27 +0000

We often discuss Java limitations on IRC and try to come up with (sometimes silly) workarounds. Unfortunately after time passes it’s often easy to forget the outcome, and lose code snippets. So I thought I’d start blogging some of them so I don’t lose them, and other people might suggest other ways of doing things that we’ve overlooked.

This particular problem occurs when you want to assign the result of a method that can throw an exception to a final variable. For example:

final Customer c;
try
{
	c = getCustomer(id);
} catch (CustomerNotFoundException e)
{
	c = createNewCustomer();
}

This will fail to compile with “variable c might already have been assigned”. Of course making c not final would solve the problem, but that’s no fun.

If we were not using Exceptions, Java provides a useful ternary operator “?:” that lets us do things like:

final Customer c = customerExists(id) ? getCustomer(id) : createNewCustomer();

Which is nice and clean, but means that getCustomer is going to have to return null or throw a RuntimeException in the case that there is no matching customer, which is undesirable. Also customerExists() may be expensive.

We could also possibly use something along the lines of

final Option<Customer> c = getCustomer();

Both of these alternatives, however, require changing the API you’re consuming, and avoiding Exceptions. It would be nice if there was an equivalent of “?:” for try/catch so that you could assign the result to a final variable. The best I can manage in Java is below, can anyone do better?

import java.lang.reflect.ParameterizedType;
 
abstract class TryCatch<T, U extends Exception>
{
	public T value()
	{
		try
		{
			return Try();
		} catch (Exception e)
		{
			if (getTypeOfU().isAssignableFrom(e.getClass()))
			{
				return Catch();
			} else
			{
				throw new RuntimeException(e);
			}
		}
	}
 
	@SuppressWarnings("unchecked")
	private Class<U> getTypeOfU()
	{
		return (Class<U>) ((ParameterizedType) getClass().getGenericSuperclass()).getActualTypeArguments()[1];
	}
 
	public abstract T Try() throws U;
 
	public abstract T Catch();
}
 
//Example
public class Main
{
	private static CustomerRepo repo = new CustomerRepo();
 
	public static void main(String[] args) 
	{
		final Customer c = new TryCatch<Customer, CustomerNotFoundException>()
		{
			public Customer Try() throws CustomerNotFoundException
			{
				System.out.println("in try");
				return repo.getCustomer(1);
			}
 
			public Customer Catch()
			{
				System.out.println("in catch");
				return repo.createCustomer();
			}
		}.value();
	}
 
}
 
class CustomerRepo
{
 
	public Customer getCustomer(int id) throws CustomerNotFoundException
	{
		throw new CustomerNotFoundException();
	}
 
	public Customer createCustomer()
	{
		return new Customer();
	}
}
 
class Customer
{
}
 
class CustomerNotFoundException extends Exception
{
}

In C# we don’t run into the same problem since readonly is much less useful than Java’s final. However, if we wanted to try/catch at the same time we can do a bit better. Here’s an alternative in C#:

using System;
 
public class Example
{
	public static void Main()
	{
		Example t = new Example();
		t.Foo();
	}
 
	public void Foo()
	{
		String result1 = this.Try(() => GetBar(true)).Catch<BarException>(() => "Caught a BarException");
		String result2 = this.Try(() => GetBar(false)).Catch<BarException>(() => "Caught a BarException");
		Console.WriteLine(result1);
		Console.WriteLine(result2);
	}
 
	public String GetBar(bool succeed)
	{
		if (succeed)
			return "Success!";
		else
			throw new BarException();
	}
}
 
public class BarException : Exception {}
 
public class Tryer<TResult>
{
	private readonly Func<TResult> toTry;
	internal Tryer(Func<TResult> toTry)
	{
		this.toTry = toTry;
	}
 
	public TResult Catch<TException>(Func<TResult> whenCaught)
		where TException : Exception
	{
		try
		{
			return toTry();
		} catch (TException)
		{
			return whenCaught();
		}
	}
}
 
namespace System
{
	public static class ProvidesTry
	{
		public static Tryer<TResult> Try<T,TResult>(this T other, Func<TResult> toTry)
		{
			return new Tryer<TResult>(toTry);
		} 
	}
}