序列化Java 8流

Question

我想使用Jackson来序列化流的输出。Jackson对序列化java.util.stream.Stream没有内在的支持，但它能够序列化java.util.Iterator。

为了说明这个问题，我想序列化这个简单的接口：

public interface SerializeMe {
    Iterator<Integer> getMyNumbers();
}

我将比较序列化List.iterator()与Stream.iterator()的输出。

public class SerializeViaList implements SerializeMe {
    @Override
    public Iterator<Integer> getMyNumbers() {
        return Arrays.asList(1, 2, 3).iterator();
    }
}

public class SerializeViaStream implements SerializeMe {
    @Override
    public Iterator<Integer> getMyNumbers() {
        return Arrays.asList(1, 2, 3).stream().iterator();
    }
}

下面的方法将演示这两个类的输出：

public static void main(String[] args) throws Exception {
    ObjectMapper mapper = new ObjectMapper();

    System.out.println("via list: " + mapper.writeValueAsString(new SerializeViaList()));
    System.out.println("via stream: " + mapper.writeValueAsString(new SerializeViaStream()));
}

它的输出是：

via list: {"myNumbers":[1,2,3]}
via stream: {"myNumbers":{}}

这说明流迭代器没有正确序列化。

有趣的是，如果我添加@JsonSerialize(as=Iterator.class)，它确实有效。

@JsonSerialize(as=Iterator.class)
public Iterator<Integer> getMyNumbers() {
    // ....
}

我不想为流可以创建的每个类型的迭代器编写一个自定义的JsonSerializer。我有什么选择？

Answer 1

Iterator只被认为是一个“附加”接口，也就是说，只有在没有为对象构建bean序列化程序的情况下才使用Iterator。不幸的是，分配器适配器确实得到了一个虚拟bean序列化器，因为这个类有一个注释.这一点也不太好，甚至看起来也不太符合预期(这不是检查员使用afaik的注释)。

当您指定@JsonSerialize(as=Iterator.class)时，您强制将解释作为一个Iterator，而IteratorSerializer工作得很好。

这是我之前编写的一个Jackson模块，它允许序列化流(也包括LongStream、IntStream或DoubleStream)，方法是按顺序序列化其内容：

public class StreamModule extends SimpleModule {
    public StreamModule() {
        super(StreamModule.class.getSimpleName());
        addSerializer(LongStream.class, new LongStreamSerializer());
        addSerializer(IntStream.class, new IntStreamSerializer());
        addSerializer(DoubleStream.class, new DoubleStreamSerializer());
    }

    @Override
    public void setupModule(SetupContext context) {
        context.addSerializers(new StreamSerializers());
        super.setupModule(context);
    }

    public static class StreamSerializers extends Serializers.Base {
        @Override
        public JsonSerializer<?> findSerializer(SerializationConfig config, JavaType type, BeanDescription beanDesc) {
            Class<?> raw = type.getRawClass();
            if (Stream.class.isAssignableFrom(raw)) {
                JavaType[] params = config.getTypeFactory().findTypeParameters(type, Stream.class);
                JavaType vt = (params == null || params.length != 1) ? TypeFactory.unknownType() : params[0];
                return new StreamSerializer<Object>(config.getTypeFactory().constructParametrizedType(Stream.class, Stream.class, vt), vt);
            }
            return super.findSerializer(config, type, beanDesc);
        }
    }

    static class StreamSerializer<T> extends StdSerializer<Stream<T>> implements ContextualSerializer {
        private final JavaType streamType;
        private final JavaType elemType;

        public StreamSerializer(JavaType streamType, JavaType elemType) {
            super(streamType);
            this.streamType = streamType;
            this.elemType = elemType;
        }

        @Override
        public JsonSerializer<?> createContextual(SerializerProvider provider, BeanProperty property) throws JsonMappingException {
            if (!elemType.hasRawClass(Object.class) && (provider.isEnabled(MapperFeature.USE_STATIC_TYPING) || elemType.isFinal())) {
                JsonSerializer<Object> elemSerializer = provider.findPrimaryPropertySerializer(elemType, property);
                return new TypedStreamSerializer<T>(streamType, elemSerializer);
            }
            return this;
        }

        @Override
        public void serialize(Stream<T> stream, JsonGenerator jgen, SerializerProvider provider) throws IOException,
                JsonGenerationException {
            jgen.writeStartArray();
            try {
                stream.forEachOrdered(elem -> {
                    try {
                        provider.defaultSerializeValue(elem, jgen);
                    } catch (IOException e) {
                        throw new WrappedIOException(e);
                    }
                });
            } catch (WrappedIOException e) {
                throw (IOException) e.getCause();
            }
            jgen.writeEndArray();
        }
    }

    static class TypedStreamSerializer<T> extends StdSerializer<Stream<T>> {
        private final JsonSerializer<T> elemSerializer;

        @SuppressWarnings("unchecked")
        public TypedStreamSerializer(JavaType streamType, JsonSerializer<?> elemSerializer) {
            super(streamType);
            this.elemSerializer = (JsonSerializer<T>) elemSerializer;
        }

        @Override
        public void serialize(Stream<T> stream, JsonGenerator jgen, SerializerProvider provider) throws IOException,
                JsonGenerationException {
            jgen.writeStartArray();
            try {
                stream.forEachOrdered(elem -> {
                    try {
                        elemSerializer.serialize(elem, jgen, provider);
                    } catch (IOException e) {
                        throw new WrappedIOException(e);
                    }
                });
            } catch (WrappedIOException e) {
                throw (IOException) e.getCause();
            }
            jgen.writeEndArray();
        }
    }

    static class IntStreamSerializer extends StdSerializer<IntStream> {
        public IntStreamSerializer() {
            super(IntStream.class);
        }

        @Override
        public void serialize(IntStream stream, JsonGenerator jgen, SerializerProvider provider) throws IOException,
                JsonGenerationException {
            jgen.writeStartArray();
            try {
                stream.forEachOrdered(value -> {
                    try {
                        jgen.writeNumber(value);
                    } catch (IOException e) {
                        throw new WrappedIOException(e);
                    }
                });
            } catch (WrappedIOException e) {
                throw (IOException) e.getCause();
            }
            jgen.writeEndArray();
        }
    }

    static class LongStreamSerializer extends StdSerializer<LongStream> {
        public LongStreamSerializer() {
            super(LongStream.class);
        }

        @Override
        public void serialize(LongStream stream, JsonGenerator jgen, SerializerProvider provider) throws IOException,
                JsonGenerationException {
            jgen.writeStartArray();
            try {
                stream.forEachOrdered(value -> {
                    try {
                        jgen.writeNumber(value);
                    } catch (IOException e) {
                        throw new WrappedIOException(e);
                    }
                });
            } catch (WrappedIOException e) {
                throw (IOException) e.getCause();
            }
            jgen.writeEndArray();
        }
    }

    static class DoubleStreamSerializer extends StdSerializer<DoubleStream> {
        public DoubleStreamSerializer() {
            super(DoubleStream.class);
        }

        @Override
        public void serialize(DoubleStream stream, JsonGenerator jgen, SerializerProvider provider) throws IOException,
                JsonGenerationException {
            jgen.writeStartArray();
            try {
                stream.forEachOrdered(value -> {
                    try {
                        jgen.writeNumber(value);
                    } catch (IOException e) {
                        throw new WrappedIOException(e);
                    }
                });
            } catch (WrappedIOException e) {
                throw (IOException) e.getCause();
            }
            jgen.writeEndArray();
        }
    }

    public static final class WrappedIOException extends RuntimeException {
        private WrappedIOException(IOException e) {
            super(e);
        }
    }
}