How do you serialize Guava's immutable collections using Protostuff?

Question

I use protostuff-runtime to serialize object graphs. Some of these objects have reference to Guava immutable collections, such as ImmutableList and ImmutableSet. Protostuff is unable to deserialize these collections out of the box, because it tries to construct an instance and then "add" elements to it from the inputStream (which fails, since the collections are immutable).

Do you know of any library / protostuff plugin that does that out of the box? If not, is there a best practice to do this myself?

I've investigated, and found that protostuff has a concept of "delegate", that lets you take control of the serialization for specific types. It seems to be the answer to my problem, but I can't seem to get it working.

Here is what I have right now:

import com.dyuproject.protostuff.Schema;
import com.dyuproject.protostuff.runtime.RuntimeSchema;
import com.google.common.base.Preconditions;
import com.google.common.collect.ImmutableList;

import javax.annotation.Nonnull;
import javax.annotation.concurrent.Immutable;

/**
 * This is the POJO I want to serialize. Note that the {@code strings} field refers to an {@link ImmutableList}.
 */
@Immutable
public class Foo {

    public static final Schema<Foo> SCHEMA = RuntimeSchema.getSchema(Foo.class);

    @Nonnull
    private final ImmutableList<String> strings;

    public Foo(ImmutableList<String> strings) {
        this.strings = Preconditions.checkNotNull(strings);
    }

    @Nonnull
    public ImmutableList<String> getStrings() {
        return strings;
    }

    @Override
    public boolean equals(Object obj) {
        if (obj instanceof Foo) {
            Foo that = (Foo) obj;
            return this.strings.equals(that.strings);
        }
        return false;
    }

    @Override
    public int hashCode() {
        return strings.hashCode();
    }

}

---

import com.dyuproject.protostuff.*;
import com.dyuproject.protostuff.runtime.Delegate;
import com.dyuproject.protostuff.runtime.RuntimeSchema;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.Lists;

import java.io.IOException;
import java.util.ArrayList;

public class ImmutableListDelegate implements Delegate<ImmutableList<?>> {

    private static final Schema<ArrayList> LIST_SCHEMA = RuntimeSchema.getSchema(ArrayList.class);

    @Override
    public WireFormat.FieldType getFieldType() {
        return WireFormat.FieldType.MESSAGE;
    }

    @Override
    public ImmutableList<?> readFrom(Input input) throws IOException {
        ArrayList<?> list = LIST_SCHEMA.newMessage();
        input.mergeObject(list, LIST_SCHEMA);
        return ImmutableList.copyOf(list);
    }

    @Override
    public void writeTo(Output output, int number, ImmutableList<?> value, boolean repeated) throws IOException {
        ArrayList<?> list = Lists.newArrayList(value);
        output.writeObject(number, list, LIST_SCHEMA, repeated);
        LIST_SCHEMA.writeTo(output, list);
    }

    @Override
    public void transfer(Pipe pipe, Input input, Output output, int number, boolean repeated) throws IOException {
        throw new UnsupportedOperationException("TODO");
    }

    @Override
    public Class<?> typeClass() {
        return ImmutableList.class;
    }
}

---

import com.dyuproject.protostuff.LinkedBuffer;
import com.dyuproject.protostuff.ProtostuffIOUtil;
import com.dyuproject.protostuff.runtime.DefaultIdStrategy;
import com.dyuproject.protostuff.runtime.RuntimeEnv;
import com.google.common.collect.ImmutableList;
import org.junit.Assert;
import org.junit.Before;
import org.junit.Test;

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;

public class ImmutableListDelegateTest {

    @Before
    public void before() {
        // registers the delegate
        if (RuntimeEnv.ID_STRATEGY instanceof DefaultIdStrategy) {
            ((DefaultIdStrategy) RuntimeEnv.ID_STRATEGY).registerDelegate(new ImmutableListDelegate());
        }
    }

    @Test
    public void testDelegate() throws IOException {
        Foo foo = new Foo(ImmutableList.of("foo"));

        Assert.assertEquals(foo, serializeThenDeserialize(foo));
    }

    private Foo serializeThenDeserialize(Foo fooToSerialize) throws IOException {
        ByteArrayOutputStream out = new ByteArrayOutputStream();
        ProtostuffIOUtil.writeDelimitedTo(out, fooToSerialize, Foo.SCHEMA, buffer());
        ByteArrayInputStream in = new ByteArrayInputStream(out.toByteArray());
        Foo fooDeserialized = Foo.SCHEMA.newMessage();
        ProtostuffIOUtil.mergeDelimitedFrom(in, fooDeserialized, Foo.SCHEMA, buffer());
        return fooDeserialized;
    }

    private LinkedBuffer buffer() {
        return LinkedBuffer.allocate(LinkedBuffer.DEFAULT_BUFFER_SIZE);
    }
}

The test fails with the following exception, which seems to mean that my delegate only deserializes null values:

java.lang.NullPointerException
    at com.google.common.base.Preconditions.checkNotNull(Preconditions.java:191)
    at com.google.common.collect.SingletonImmutableList.<init>(SingletonImmutableList.java:40)
    at com.google.common.collect.ImmutableList.asImmutableList(ImmutableList.java:305)
    at com.google.common.collect.ImmutableList.copyFromCollection(ImmutableList.java:314)
    at com.google.common.collect.ImmutableList.copyOf(ImmutableList.java:253)
    at test.ImmutableListDelegate.readFrom(ImmutableListDelegate.java:25)
    at test.ImmutableListDelegate.readFrom(ImmutableListDelegate.java:12)
    at com.dyuproject.protostuff.runtime.RuntimeUnsafeFieldFactory$19$1.mergeFrom(RuntimeUnsafeFieldFactory.java:1111)
    at com.dyuproject.protostuff.runtime.MappedSchema.mergeFrom(MappedSchema.java:188)
    at com.dyuproject.protostuff.IOUtil.mergeDelimitedFrom(IOUtil.java:109)
    at com.dyuproject.protostuff.ProtostuffIOUtil.mergeDelimitedFrom(ProtostuffIOUtil.java:151)
    at test.ImmutableListDelegateTest.serializeThenDeserialize(ImmutableListDelegateTest.java:38)
    at test.ImmutableListDelegateTest.testDelegate(ImmutableListDelegateTest.java:30)

Is this the right approach? What am I missing?

This is not a duplicate of the What is a Null Pointer Exception, and how do I fix it? question, which makes no sense. The fact that I mentioned that an NPE is thrown when trying to use a Protostuff delegate to de-serialize immutable collections, doesn't mean that this duplicates the "What is a NPE?" question in any way, shape, or form.

Answer 1

Everything looks fine and the

java.lang.NullPointerException
    at com.google.common.base.Preconditions.checkNotNull(Preconditions.java:191)
    at com.google.common.collect.SingletonImmutableList.<init>(SingletonImmutableList.java:40)

says that you're trying to put null into an ImmutableList, which is forbidden. To be sure, inspect you list just before the failing line. Make sure your input json doesn't look like [null].