Jackson: Difference between revisions
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Revision as of 01:55, 14 November 2018
External
- Jackson Documentation Hub: https://github.com/FasterXML/jackson-docs
- Jackson JSON Processor Wiki http://wiki.fasterxml.com/JacksonHome
- Jackson in Five Minutes http://wiki.fasterxml.com/JacksonInFiveMinutes
- Jackson JSON Tutorial http://www.baeldung.com/jackson
- javadocs:
- Jackson Core https://github.com/FasterXML/jackson-core/wiki
- Jackson Core Annotations https://github.com/FasterXML/jackson-annotations/wiki
- Jackson Databind https://github.com/FasterXML/jackson-databind/wiki
Internal
Overview
Jackson is a Java library for processing JSON data format. It has support for marshaling and unmarshalling Java to and from JSON. It has a JAX-RS content handler that can automatically convert between Java objects and JSON, and vice-versa. It can generate JSON schemas from a Java object model.
JSON Processing Methods
Jackson offers three alternative methods for processing JSON:
Streaming API
The Streaming API reads and writes JSON as a series of discrete events, in a mode called "incremental parsing/generation". The other two methods (Tree Model and Data Binding) are built in top of it. However, this is not the most convenient method, because is relatively low level. For more details, see:
Tree Model
A JSON document is converted into a mutable in-memory tree representation of the JSON document. Tree model is arguably the most flexible of all three methods. For more details, see:
Data Binding
Jackson extracts data from JSON and initializes a in-memory Java object hierarchy. It uses non-Jackson types in the process, unlike the tree model, where JSON is translated to a JsonNode hierarchy.
In case of simple data binding, those Java types are Maps, Lists, Strings, Numbers, Booleans and nulls. In case of full data binding, Jackson converts JSON to any Java bean type. Data binding is arguably the most convenient of all three methods.
Simple Data Binding
Full Data Binding
ObjectMapper
Thread Safety
ObjectMapper is thread safe, so it does not have to be created for each request that needs serialization/deserialization.
It is recommended to declare a static ObjectMapper instance that should be shared as much as possible.
The only problem with this approach is that the configuration of the mapper cannot be changed after it was shared, otherwise clients will rely on invalid assumptions,
With 2.0 and above, ObjectWriter and ObjectReader can be constructed by ObjectMapper. These objects are fully immutable, thread-safe and cheap to create?
Configuration
Mapper Features
Annotation Use
ObjectMapper can be configured to use or not annotation introspection. By default is true.
om.configure(MapperFeature.USE_ANNOTATIONS, true);
Use Getters as Setters
This feature determines whether the "getter" methods that handle Collections and Maps can be used for purpose of getting a reference to a Collection and Map to modify the property, without requiring a setter method. This is similar to how JAXB framework sets Collections and Maps. Note that such getters-as-setters methods have lower precedence than setters, so they are only used if no setter is found for the Map/Collection property. The feature is enabled by default.
om.configure(MapperFeature.USE_GETTERS_AS_SETTERS, true);
Propagate Transient Marker
This feature determines how transient modifier for fields is handled: if disabled, it is only taken to mean exclusion of the field as accessor; if true, it is taken to imply removal of the whole property. Feature is disabled by default, meaning that existence of transient for a field does not cause Jackson to ignore getters or setters just ignoring the use of field for access.
om.configure(MapperFeature.PROPAGATE_TRANSIENT_MARKER, false);
Deserialization Features
Fail on Ignored Properties
This feature determines what happens when a property that has been explicitly marked as ignorable is encountered in input: if feature is enabled, JsonMappingException is thrown; if false, property is quietly skipped. The feature is disabled by default so that no exception is thrown. Another way to look at it is as configuration to ignore fields that appear in the JSON serialized format, but are not declared as members in the type. The same effect can be achieved by annotating the type with @JsonIgnoreProperties
om.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
Specifying a Date Format
ISO 8601 date and time format can be automatically handled (converted from and to) by the ObjectMapper, as follows:
import com.fasterxml.jackson.databind.SerializationFeature;
import com.fasterxml.jackson.databind.util.StdDateFormat;
objectMapper.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);
objectMapper.setDateFormat(new StdDateFormat());
More:
Annotations
Maven Support
<dependency> <groupId>com.fasterxml.jackson.core</groupId> <artifactId>jackson-core</artifactId> <version>2.8.7</version> </dependency>
If the tree model is used, this is also required:
<dependency> <groupId>com.fasterxml.jackson.core</groupId> <artifactId>jackson-databind</artifactId> <version>2.8.7</version> </dependency>