how to deserialize / serialize byte array using Jackson and wrapper object

Question

I have two following classes:

public class User {

    private String name;

    private Secret secret;

    public User( @JsonProperty("name") String name, @JsonProperty("secret") Secret secret ) {
        this.name = name;
        this.secret = secret;
    }

    public String getName() {
        return name;
    }

    public Secret getSecret() {
        return secret;
    }

}

and

public class Secret {

    private byte[] secret;

    public Secret( byte[] secret ) {
        this.secret = secret;
    }

    @JsonValue
    public byte[] getSecret() {
        return secret;
    }

}

I would like to use these classes to serialize / deserialize following json:

{
  "name": "bdf",
  "secret": "AQ=="
}

Java to json works properly. However when I try to deserialize json I get the following exception:

com.fasterxml.jackson.databind.JsonMappingException: Can not instantiate value of type [simple type, class deserialization.Secret] from String value ('YQ=='); no single-String constructor/factory method
 at [Source: [B@3b938003; line: 1, column: 25] (through reference chain: deserialization.User["secret"])
    at com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:148)
    at com.fasterxml.jackson.databind.DeserializationContext.mappingException(DeserializationContext.java:875)
    at com.fasterxml.jackson.databind.deser.ValueInstantiator._createFromStringFallbacks(ValueInstantiator.java:281)
    at com.fasterxml.jackson.databind.deser.std.StdValueInstantiator.createFromString(StdValueInstantiator.java:284)
    at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.deserializeFromString(BeanDeserializerBase.java:1176)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer._deserializeOther(BeanDeserializer.java:143)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:134)
    at com.fasterxml.jackson.databind.deser.SettableBeanProperty.deserialize(SettableBeanProperty.java:520)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer._deserializeWithErrorWrapping(BeanDeserializer.java:461)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer._deserializeUsingPropertyBased(BeanDeserializer.java:376)
    at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.deserializeFromObjectUsingNonDefault(BeanDeserializerBase.java:1099)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:294)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:131)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:3702)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2807)
    at deserialization.SerializationTest.itShouldDeserialize(SerializationTest.java:22)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:675)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)

How can I tell jackson to first decode base64 encoded value and then use this constructor?

public Secret( byte[] secret )

Answer 1

This can be done by writing your own custom serializer and deserializer.

http://www.baeldung.com/jackson-custom-serialization

http://www.baeldung.com/jackson-deserialization

Here's an example similar to what you are doing...

Holder class

public static class Holder {
    private Bytes bytes;
    private String otherStuff;

    public Bytes getBytes() {
        return bytes;
    }

    public void setBytes(Bytes bytes) {
        this.bytes = bytes;
    }

    public String getOtherStuff() {
        return otherStuff;
    }

    public void setOtherStuff(String otherStuff) {
        this.otherStuff = otherStuff;
    }

}

Bytes class

Notice the annotations for the custom serializer...

@JsonSerialize(using = BytesSerializer.class)
@JsonDeserialize(using = BytesDeserializer.class)
public static class Bytes {
    private byte[] bytes;

    public Bytes(byte[] bytes) {
        this.bytes = bytes;
    }

    public byte[] getBytes() {
        return bytes;
    }
}

Serializer

This will serialize a "Bytes" object as a base64 string...

public static class BytesSerializer extends StdSerializer<Bytes> {

    private static final long serialVersionUID = -5510353102817291511L;

    public BytesSerializer() {
        super(Bytes.class);
    }

    @Override
    public void serialize(Bytes value, JsonGenerator gen, SerializerProvider provider) throws IOException {
        gen.writeString(Base64.encode(value.getBytes()));
    }
}

Deserializer

public static class BytesDeserializer extends StdDeserializer<Bytes> {

    private static final long serialVersionUID = 1514703510863497028L;

    public BytesDeserializer() {
        super(Bytes.class);
    }

    @Override
    public Bytes deserialize(JsonParser p, DeserializationContext ctxt) throws IOException, JsonProcessingException {
        JsonNode node = p.getCodec().readTree(p);
        String base64 = node.asText();
        return new Bytes(Base64.decode(base64));
    }
}

Main method

A simple test method...

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

    Holder holder = new Holder();
    holder.setOtherStuff("[OTHER STUFF]");
    holder.setBytes(new Bytes(new byte[] { 1, 2, 3, 4, 5 }));

    String json = mapper.writeValueAsString(holder);

    System.out.println(json);
    Holder deserialised = mapper.readValue(json, Holder.class);

    System.out.println(Arrays.toString(deserialised.getBytes().getBytes()));
}

Output

{"bytes":"AQIDBAU=","otherStuff":"[OTHER STUFF]"}
[1, 2, 3, 4, 5]

Run it yourself

All of the classes above are "static" because I'd wrapped them into one big class called "Stack".

If you want to run this, create a new class (called anything you want) and paste all the code here into it...