JarCastingOverview
This project contains the general-purpose data-binding functionality and tree-model for Jackson Data Processor. It builds on Streaming API (stream parser/generator) package, and uses Jackson Annotations for configuration. Project is licensed under Apache License 2.0.
While the original use case for Jackson was JSON data-binding, it can now be used to read content encoded in other data formats as well, as long as parser and generator implementations exist. Naming of classes uses word 'JSON' in many places even though there is no actual hard dependency to JSON format.
Get it!
Maven
Functionality of this package is contained in Java package com.fasterxml.jackson.databind, and can be used using following Maven dependency:
<properties>
...
<!-- Use the latest version whenever possible. -->
<jackson.version>2.12.0</jackson.version>
...
</properties>
<dependencies>
...
<dependency>
<groupId>com.fasterxml.jackson.core</groupId>
<artifactId>jackson-databind</artifactId>
<version>${jackson.version}</version>
</dependency>
...
</dependencies>
Package also depends on jackson-core and jackson-annotations packages, but when using build tools like Maven or Gradle, dependencies are automatically included. You may, however, want to use jackson-bom to ensure compatible versions of dependencies. If not using build tool that can handle dependencies using project's pom.xml, you will need to download and include these 2 jars explicitly.
Non-Maven dependency resolution
For use cases that do not automaticall resolve dependencies from Maven repositories, you can still download jars from Central Maven repository.
Databind jar is also a functional OSGi bundle, with proper import/export declarations, so it can be use on OSGi container as is.
With Jackson 2.10, jar will also include module-info.class to work as proper Java Module.
Jackson 2.12 also includes additional Gradle 6 Module Metadata for version alignment with Gradle.
Use It!
More comprehensive documentation can be found from Jackson-docs repository; as well as from Wiki of this project. But here are brief introductionary tutorials, in recommended order of reading.
1 minute tutorial: POJOs to JSON and back
The most common usage is to take piece of JSON, and construct a Plain Old Java Object ("POJO") out of it. So let's start there. With simple 2-property POJO like this:
// Note: can use getters/setters as well; here we just use public fields directly:
public class MyValue {
public String name;
public int age;
// NOTE: if using getters/setters, can keep fields `protected` or `private`
}
we will need a com.fasterxml.jackson.databind.ObjectMapper instance, used for all data-binding, so let's construct one:
ObjectMapper mapper = new ObjectMapper(); // create once, reuse
The default instance is fine for our use -- we will learn later on how to configure mapper instance if necessary. Usage is simple:
MyValue value = mapper.readValue(new File("data.json"), MyValue.class);
// or:
value = mapper.readValue(new URL("http://some.com/api/entry.json"), MyValue.class);
// or:
value = mapper.readValue("{\"name\":\"Bob\", \"age\":13}", MyValue.class);
And if we want to write JSON, we do the reverse:
mapper.writeValue(new File("result.json"), myResultObject);
// or:
byte[] jsonBytes = mapper.writeValueAsBytes(myResultObject);
// or:
String jsonString = mapper.writeValueAsString(myResultObject);
So far so good?
3 minute tutorial: Generic collections, Tree Model
Beyond dealing with simple Bean-style POJOs, you can also handle JDK Lists, Maps:
Map<String, Integer> scoreByName = mapper.readValue(jsonSource, Map.class);
List<String> names = mapper.readValue(jsonSource, List.class);
// and can obviously write out as well
mapper.writeValue(new File("names.json"), names);
as long as JSON structure matches, and types are simple. If you have POJO values, you need to indicate actual type (note: this is NOT needed for POJO properties with List etc types):
Map<String, ResultValue> results = mapper.readValue(jsonSource,
new TypeReference<Map<String, ResultValue>>() { } );
// why extra work? Java Type Erasure will prevent type detection otherwise
(note: no extra effort needed for serialization, regardless of generic types)
But wait! There is more!
While dealing with Maps, Lists and other "simple" Object types (Strings, Numbers, Booleans) can be simple, Object traversal can be cumbersome. This is where Jackson's Tree model can come in handy:
// can be read as generic JsonNode, if it can be Object or Array; or,
// if known to be Object, as ObjectNode, if array, ArrayNode etc:
ObjectNode root = mapper.readTree("stuff.json");
String name = root.get("name").asText();
int age = root.get("age").asInt();
// can modify as well: this adds child Object as property 'other', set property 'type'
root.with("other").put("type", "student");
String json = mapper.writeValueAsString(root);
// with above, we end up with something like as 'json' String:
// {
// "name" : "Bob", "age" : 13,
// "other" : {
// "type" : "student"
// }
// }
Tree Model can be more convenient than data-binding, especially in cases where structure is highly dynamic, or does not map nicely to Java classes.
5 minute tutorial: Streaming parser, generator
As convenient as data-binding (to/from POJOs) can be; and as flexible as Tree model can be, there is one more canonical processing model available: incremental (aka "streaming") model. It is the underlying processing model that data-binding and Tree Model both build upon, but it is also exposed to users who want ultimate performance and/or control over parsing or generation details.
For in-depth explanation, look at Jackson Core component. But let's look at a simple teaser to whet your appetite.
JsonFactory f = mapper.getFactory(); // may alternatively construct directly too
// First: write simple JSON output
File jsonFile = new File("test.json");
JsonGenerator g = f.createGenerator(jsonFile);
// write JSON: { "message" : "Hello world!" }
g.writeStartObject();
g.writeStringField("message", "Hello world!");
g.writeEndObject();
g.close();
// Second: read file back
JsonParser p = f.createParser(jsonFile);
JsonToken t = p.nextToken(); // Should be JsonToken.START_OBJECT
t = p.nextToken(); // JsonToken.FIELD_NAME
if ((t != JsonToken.FIELD_NAME) || !"message".equals(p.getCurrentName())) {
// handle error
}
t = p.nextToken();
if (t != JsonToken.VALUE_STRING) {
// similarly
}
String msg = p.getText();
System.out.printf("My message to you is: %s!\n", msg);
p.close();
10 minute tutorial: configuration
There are two entry-level configuration mechanisms you are likely to use: Features and Annotations.
Commonly used Features
Here are examples of configuration features that you are most likely to need to know about.
Let's start with higher-level data-binding configuration.
// SerializationFeature for changing how JSON is written
// to enable standard indentation ("pretty-printing"):
mapper.enable(SerializationFeature.INDENT_OUTPUT);
// to allow serialization of "empty" POJOs (no properties to serialize)
// (without this setting, an exception is thrown in those cases)
mapper.disable(SerializationFeature.FAIL_ON_EMPTY_BEANS);
// to write java.util.Date, Calendar as number (timestamp):
mapper.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);
// DeserializationFeature for changing how JSON is read as POJOs:
// to prevent exception when encountering unknown property:
mapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);
// to allow coercion of JSON empty String ("") to null Object value:
mapper.enable(DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT);
In addition, you may need to change some of low-level JSON parsing, generation details:
// JsonParser.Feature for configuring parsing settings:
// to allow C/C++ style comments in JSON (non-standard, disabled by default)
// (note: with Jackson 2.5, there is also `mapper.enable(feature)` / `mapper.disable(feature)`)
mapper.configure(JsonParser.Feature.ALLOW_COMMENTS, true);
// to allow (non-standard) unquoted field names in JSON:
mapper.configure(JsonParser.Feature.ALLOW_UNQUOTED_FIELD_NAMES, true);
// to allow use of apostrophes (single quotes), non standard
mapper.configure(JsonParser.Feature.ALLOW_SINGLE_QUOTES, true);
// JsonGenerator.Feature for configuring low-level JSON generation:
// to force escaping of non-ASCII characters:
mapper.configure(JsonGenerator.Feature.ESCAPE_NON_ASCII, true);
Full set of features are explained on Jackson Features page.
Annotations: changing property names
The simplest annotation-based approach is to use @JsonProperty annotation like so:
public class MyBean {
private String _name;
// without annotation, we'd get "theName", but we want "name":
@JsonProperty("name")
public String getTheName() { return _name; }
// note: it is enough to add annotation on just getter OR setter;
// so we can omit it here
public void setTheName(String n) { _name = n; }
}
There are other mechanisms to use for systematic naming changes: see Custom Naming Convention for details.
Note, too, that you can use Mix-in Annotations to associate all annotations.
Annotations: Ignoring properties
There are two main annotations that can be used to to ignore properties: @JsonIgnore for individual properties; and @JsonIgnoreProperties for per-class definition
// means that if we see "foo" or "bar" in JSON, they will be quietly skipped
// regardless of whether POJO has such properties
@JsonIgnoreProperties({ "foo", "bar" })
public class MyBean
{
// will not be written as JSON; nor assigned from JSON:
@JsonIgnore
public String internal;
// no annotation, public field is read/written normally
public String external;
@JsonIgnore
public void setCode(int c) { _code = c; }
// note: will also be ignored because setter has annotation!
public int getCode() { return _code; }
}
As with renaming, note that annotations are "shared" between matching fields, getters and setters: if only one has @JsonIgnore, it affects others. But it is also possible to use "split" annotations, to for example:
public class ReadButDontWriteProps {
private String _name;
@JsonProperty public void setName(String n) { _name = n; }
@JsonIgnore public String getName() { return _name; }
}
in this case, no "name" property would be written out (since 'getter' is ignored); but if "name" property was found from JSON, it would be assigned to POJO property!
For a more complete explanation of all possible ways of ignoring properties when writing out JSON, check "Filtering properties" article.
Annotations: using custom constructor
Unlike many other data-binding packages, Jackson does not require you to define "default constructor" (constructor that does not take arguments). While it will use one if nothing else is available, you can easily define that an argument-taking constructor is used:
public class CtorBean
{
public final String name;
public final int age;
@JsonCreator // constructor can be public, private, whatever
private CtorBean(@JsonProperty("name") String name,
@JsonProperty("age") int age)
{
this.name = name;
this.age = age;
}
}
Constructors are especially useful in supporting use of Immutable objects.
Alternatively, you can also define "factory methods":
public class FactoryBean
{
// fields etc omitted for brewity
@JsonCreator
public static FactoryBean create(@JsonProperty("name") String name) {
// construct and return an instance
}
}
Note that use of a "creator method" (@JsonCreator with @JsonProperty annotated arguments) does not preclude use of setters: you can mix and match properties from constructor/factory method with ones that are set via setters or directly using fields.
Tutorial: fancier stuff, conversions
One useful (but not very widely known) feature of Jackson is its ability to do arbitrary POJO-to-POJO conversions. Conceptually you can think of conversions as sequence of 2 steps: first, writing a POJO as JSON, and second, binding that JSON into another kind of POJO. Implementation just skips actual generation of JSON, and uses more efficient intermediate representation.
Conversions work between any compatible types, and invocation is as simple as:
ResultType result = mapper.convertValue(sourceObject, ResultType.class);
and as long as source and result types are compatible -- that is, if to-JSON, from-JSON sequence would succeed -- things will "just work". But here are couple of potentially useful use cases:
// Convert from List<Integer> to int[]
List<Integer> sourceList = ...;
int[] ints = mapper.convertValue(sourceList, int[].class);
// Convert a POJO into Map!
Map<String,Object> propertyMap = mapper.convertValue(pojoValue, Map.class);
// ... and back
PojoType pojo = mapper.convertValue(propertyMap, PojoType.class);
// decode Base64! (default byte[] representation is base64-encoded String)
String base64 = "TWFuIGlzIGRpc3Rpbmd1aXNoZWQsIG5vdCBvbmx5IGJ5IGhpcyByZWFzb24sIGJ1dCBieSB0aGlz";
byte[] binary = mapper.convertValue(base64, byte[].class);
Basically, Jackson can work as a replacement for many Apache Commons components, for tasks like base64 encoding/decoding, and handling of "dyna beans" (Maps to/from POJOs).
Tutorial: Builder design pattern + Jackson
The Builder design pattern is a creational design pattern and can be used to create complex objects step by step. If we have an object that needs multiple checks on other dependencies, In such cases, it is preferred to use builder design pattern.
Let's consider the person structure, which has some optional fields
public class Person {
private final String name;
private final Integer age;
// getters
}
Let’s see how we can employ its power in deserialization. First of all, let’s declare a private all-arguments constructor, and a Builder class.
private Person(String name, Integer age) {
this.name = name;
this.age = age;
}
static class Builder {
String name;
Integer age;
Builder withName(String name) {
this.name = name;
return this;
}
Builder withAge(Integer age) {
this.age = age;
return this;
}
public Person build() {
return new Person(name, age);
}
}
First of all, we need to mark our class with @JsonDeserialize annotation, passing a builder parameter with a fully qualified domain name of a builder class. After that, we need to annotate the builder class itself as @JsonPOJOBuilder.
@JsonDeserialize(builder = Person.Builder.class)
public class Person {
//...
@JsonPOJOBuilder
static class Builder {
//...
}
}
A simple unit test will be:
String json = "{\"name\":\"Hassan\",\"age\":23}";
Person person = new ObjectMapper().readValue(json, Person.class);
assertEquals("Hassan", person.getName());
assertEquals(23, person.getAge().intValue());
If your builder pattern implementation uses other prefixes for methods or uses other names than build() for the builder method Jackson also provide a handy way for you.
For example, if you have a builder class uses the "set" prefix for its methods and use the create() method instead of build() for building the whole class, you have to annotate your class like:
@JsonPOJOBuilder(buildMethodName = "create", withPrefix = "set")
static class Builder {
String name;
Integer age;
Builder setName(String name) {
this.name = name;
return this;
}
Builder setAge(Integer age) {
this.age = age;
return this;
}
public Person create() {
return new Person(name, age);
}
}
Overall, Jackson library is very powerful in deserializing objects using builder pattern.
Contribute!
We would love to get your contribution, whether it's in form of bug reports, Requests for Enhancement (RFE), documentation, or code patches. The primary mechanism for all of above is GitHub Issues system.
Basic rules for Code Contributions
There is really just one main rule, which is that to accept any code contribution, we need to get a filled Contributor License Agreement (CLA) from the author. One CLA is enough for any number of contributions, but we need one. Or, rather, companies that use our code want it. It keeps their lawyers less unhappy about Open Source usage.
Limitation on Dependencies by Core Components
One additional limitation exists for so-called core components (streaming api, jackson-annotations and jackson-databind): no additional dependencies are allowed beyond:
- Core components may rely on any methods included in the supported JDK
- Minimum JDK version was 1.5 until (and including) version 2.3
- Minimum JDK version was 1.6 for Jackson 2.4 - 2.7 (inclusive) for all core components
- Minimum is still 1.6 for
jackson-annotationsandjackson-core, for all remaining Jackson 2.x versions
- Minimum is still 1.6 for
- Minimum JDK version is 1.7 for Jackson 2.7 - 2.10 of
jackson-databindand most non-core components
- Jackson-databind (this package) depends on the other two (annotations, streaming).
This means that anything that has to rely on additional APIs or libraries needs to be built as an extension, usually a Jackson module.
Branches
master branch is for developing the next major Jackson version -- 3.0 -- but there are active maintenance branches in which much of development happens:
2.12is the branch for "next" minor version to release (as of October 2020)2.11is the current stable minor 2.x version2.10is for selected backported fixes2.9may still get security fixes (until December 31, 2020)
Older branches are usually not maintained after being declared as closed on Jackson Releases page, but exist just in case a rare emergency patch is needed. All released versions have matching git tags (jackson-dataformats-binary-2.9.10).
Differences from Jackson 1.x
Project contains versions 2.0 and above: source code for last (1.x) release, 1.9, is available at Jackson-1 repo.
Main differences compared to 1.x "mapper" jar are:
- Maven build instead of Ant
- Java package is now
com.fasterxml.jackson.databind(instead oforg.codehaus.jackson.map)
Support
Community support
Jackson components are supported by the Jackson community through mailing lists, Gitter forum, Github issues. See Participation, Contributing for full details.
Enterprise support
Available as part of the Tidelift Subscription.
The maintainers of jackson-databind and thousands of other packages are working with Tidelift to deliver commercial support and maintenance for the open source dependencies you use to build your applications. Save time, reduce risk, and improve code health, while paying the maintainers of the exact dependencies you use. Learn more.
Further reading
Related:
- Core annotations package defines annotations commonly used for configuring databinding details
- Core parser/generator package defines low-level incremental/streaming parsers, generators
- Jackson Project Home has links to all modules
- Jackson Docs is project's documentation hub
