public class PrivateSetConventionTest {
    
    public static class Pojo {
        protected String protectedFoo;
        private String   privateFoo;
        String ppFoo;
        public String getProtectedFoo() {
            return protectedFoo;
        }
        public String getPrivateFoo() {
            return privateFoo;
        }        @Override
        public boolean equals( Object o ) {
            if ( this == o ) return true;
            if ( !( o instanceof Pojo ) ) return false;
            Pojo pojo = ( Pojo ) o;
            return Objects.equals( protectedFoo, pojo.protectedFoo ) &&
                   Objects.equals( privateFoo, pojo.privateFoo ) &&
                   Objects.equals( ppFoo, pojo.ppFoo );
        }        @Override
        public int hashCode() {
            return Objects.hash( protectedFoo, privateFoo, ppFoo );
        }        @Override
        public String toString() {
            return "Pojo{" + "protectedFoo='" + protectedFoo + '\'' + ", privateFoo='" + privateFoo + '\'' +
                   ", ppFoo='" + ppFoo + '\'' + '}';
        }
    }    private CodecRegistry codecRegistry;    @BeforeClass
    public void setup() {
        List<Convention> conventions = Arrays.asList( Conventions.SET_PRIVATE_FIELDS_CONVENTION );
        CodecProvider pojoCodecProvider = PojoCodecProvider.builder().automatic(true).conventions( conventions ).build();
        codecRegistry = CodecRegistries.fromRegistries(
                MongoClientSettings.getDefaultCodecRegistry(),
                CodecRegistries.fromProviders( pojoCodecProvider )
        );
    }    @Test
    public void testDecode() {
        Pojo expected = new Pojo();
        expected.protectedFoo = "Test 1";
        expected.privateFoo = "Test 2";
        expected.ppFoo = "Test 3";
        BsonDocument rawBson = new BsonDocument().append( "protectedFoo", new BsonString( expected.protectedFoo ) )
                                                 .append( "privateFoo", new BsonString( expected.privateFoo ) )
                                                 .append( "ppFoo", new BsonString( expected.ppFoo ) );
        Codec<Pojo> pojoCodec = codecRegistry.get( Pojo.class );
        BsonDocumentReader reader = new BsonDocumentReader( rawBson );
        Pojo actual = pojoCodec.decode( reader, DecoderContext.builder().build() );
        Assert.assertEquals( actual, expected );
    }
}

Expected :Pojo{protectedFoo='Test 1', privateFoo='Test 2', ppFoo='Test 3'}
Actual   :Pojo{protectedFoo='null', privateFoo='Test 2', ppFoo='null'}