Java Gson Exclude fields during serialization

Question

I have a ConfigInstance class which contains a password and a password_hash . Now I want to serialize the object using gson but exclude the password field.

public class ConfigInstance {
    public String database_address;
    public int database_port;
    public String database_user;

    @Expose(serialize = false)
    private String database_pass;
    public String database_pass_hash;

    public String GetPass() { return database_pass; }

    public void Encrypt() { /* Creates the hash before serializing*/ }

    public void Decrypt() { /* Creates the password after deserializing */}
}

As you can see, I have tried using @Expose(serialize = false) but it doesn't seem to do anything. Also I already set the field to private since I figured that this would "override" the @Expose

but running following code:

private void toFile(File file, ConfigInstance map) {
    map.Encrypt();
    Gson gson = new GsonBuilder().setPrettyPrinting().create();
    String jsonConfig = gson.toJson(map);
    FileWriter writer;
    try {
        writer = new FileWriter(file);
        writer.write(jsonConfig);
        writer.flush();
        writer.close();
    } catch (IOException e) {
        System.out.println("Error exporting config: " + e.toString());
    }
}

still results in the following file content without Errors:

{
  "database_address": "127.0.0.1",
  "database_port": 1521,
  "database_user": "test",
  "database_pass": "test1234",
  "database_pass_hash": "B9FE2C011B59F0D0D383D70073E48A19"
}

So what am I doing wrong? I am pretty clueless right now and would appreciate any help since THIS doesn't seem to work.

Thanks in advance.

Answer 1

In order to get this result, you need to annotate all the fields with the @Expose :

public class ConfigInstance {

    @Expose
    public String database_address;
    @Expose
    public int database_port;
    @Expose
    public String database_user;

    @Expose(serialize = false)
    private String database_pass;
    @Expose
    public String database_pass_hash;

And configure Gson to only expose fields that are annotated and ignore the rest as shown in the following:

Gson gson = new GsonBuilder().excludeFieldsWithoutExposeAnnotation().setPrettyPrinting().create();

Then, you'll get:

{
  "database_address": "127.0.0.1",
  "database_port": 1521,
  "database_user": "test",
  "database_pass_hash": "B9FE2C011B59F0D0D383D70073E48A19"
}

Also, when deserialising the string you'll still have the password attribute as well.

---

Still, you have the possibility to configure a Gson Serializer to accomplish this.

Answer 2

This is another way.

serialization:

Gson gson = new GsonBuilder()
            .addSerializationExclusionStrategy(new ExclusionStrategy() {
                @Override
                public boolean shouldSkipField(FieldAttributes f) {
                    return f.getName().toLowerCase().contains("fieldName");
                }

                @Override
                public boolean shouldSkipClass(Class<?> aClass) {
                    return false;
                }
            })
            .create();

deserialization:

Gson gson = new GsonBuilder()
            .addDeserializationExclusionStrategy(new ExclusionStrategy() {
                @Override
                public boolean shouldSkipField(FieldAttributes f) {
                    return f.getName().toLowerCase().contains("fieldName");
                }

                @Override
                public boolean shouldSkipClass(Class<?> aClass) {
                    return false;
                }
            })
            .create();

Answer 3

If you like particular field not be serialized give it a transient keyword

private transient String database_pass;

visit https://sites.google.com/site/gson/gson-user-guide#TOC-Finer-Points-with-Objects for more information

Answer 4

@utkusonmez This answer works albeit the method mentioned is wrong. It should be using 'addSerializationExclusionStrategy' instead of 'addDeserializationExclusionStrategy'

So the answer would look like

Gson gson = new GsonBuilder()
            .addSerializationExclusionStrategy(new ExclusionStrategy() {
                @Override
                public boolean shouldSkipField(FieldAttributes f) {
                    return f.getName().toLowerCase().contains("fieldName");
                }

                @Override
                public boolean shouldSkipClass(Class<?> aClass) {
                    return false;
                }
            })
            .create();

gson.toJson(*OBJ_TO_SERIALIZE*))

Answer 5

If you want to disable only serialization or only deserialization, you can play with @Expose annotation's attributes, eg:

@Expose(serialize = false, deserialize = true)

Default option is true, so deserialize is unnecessary here.

Answer 6

It's a late answer but it may help someone.

You only have to do the @Expose(serialize = false, deserialize = false) or just what you want.

If you have serialize and deserialize true its not nesseccary to have @Expose at all.

Create this Class:

import com.google.gson.ExclusionStrategy;
import com.google.gson.FieldAttributes;
import com.google.gson.annotations.Expose;

public class NoModuleExclusionStrategy implements ExclusionStrategy {

    private final boolean deserialize;

    public NoModuleExclusionStrategy(boolean isdeserialize) {
        deserialize = isdeserialize;
    }

    @Override
    public boolean shouldSkipClass(Class<?> clazz) {
        return false;
    }

    @Override
    public boolean shouldSkipField(FieldAttributes field) {
        return !(field.getAnnotation(Expose.class) == null || (deserialize ? field.getAnnotation(Expose.class).deserialize() : field.getAnnotation(Expose.class).serialize()));
    }

}

and then build Gson with GsonBuilder

Gson gson = new GsonBuilder()
.addSerializationExclusionStrategy(new NoModuleExclusionStrategy(false))
.addDeserializationExclusionStrategy(new NoModuleExclusionStrategy(true))
.create();

Your ConfigInstance Class will look like follows

{
  "database_address": "127.0.0.1",
  "database_port": 1521,
  "database_user": "test",
  "database_pass_hash": "B9FE2C011B59F0D0D383D70073E48A19"
}

Answer 7

Here's a very simple solution with just one line of code, without any @Expose annotation or exclusion strategy.

The default behaviour that is implemented in Gson is that null object fields are ignored . So, all you need to do is set the password field to null before serializing.

map.setPassword(null); // or map.password = null;
String jsonConfig = gson.toJson(map); // This won't serialize null password field