# Overview

Jackson jr is a compact alternative to full [Jackson Databind](../../../jackson-databind) component.
It implements a subset of functionality, for example for cases where:

1. Size of jar matters (`jackson-jr-objects` is bit over 100 kB)
2. Startup time matters (`jackson-jr` has very low initialization overhead)

In addition to basic datatypes (core JDK types like `List`s, `Map`s, wrapper types),
package supports reading and writing of standard Java Beans (implementation that mimics standard
JDK Bean Introspection): that is,
subset of POJOs that define setters/getters and/or `public` fields.
And with 2.11 there is even optional support for a subset of Jackson annotations
via optional `jackson-jr-annotatin-support` extension.

Jackson-jr also adds  `composer` implementation that can be used to
construct JSON output with builder-style API, but without necessarily having
to build an in-memory representation: instead, it can directly use `streaming-api`
for direct output. It is also possible to build actual in-memory
JSON `String` or `byte[]` representation, if that is preferable.

Main Jackson-jr artifact (`jackson-jr-objects`) itself is currently about 120 kB in size, and only depends on
[Jackson Streaming API](../../../jackson-core) package.
Combined size, for "all" jar, is bit over 500 kB (of which streaming API is about 350 kB),
for use cases where a single jar is preferred over more modular approach.
Finally, use of jar minimizers like [ProGuard](http://proguard.sourceforge.net/) can bring the jar
size down even further, by renaming and removing debug information.

## License

Good old [Apache License](http://www.apache.org/licenses/LICENSE-2.0).

## Packaging

Project is composed of multiple Maven sub-modules, each corresponding to a jar:

* [jr-objects](../../tree/master/jr-objects) contains the "core" databinding implementation, and is commonly the only dependency to use
    * Depends on `jackson-core` for low-level reading/writing
* [jr-stree](../../tree/master/jr-stree) contains a simple `TreeCodec` implementation, with which it is possible to read JSON as `TreeNode`s (see more below)
* [jr-retrofit2](../../tree/master/jr-retrofit2) contains `jackson-jr` - based handlers for [Retrofit 2](http://square.github.io/retrofit/) library
    * Depends on `jackson-jr` and `Retrofit` API jars, and indirectly on `jackson-core`
* [jr-annotation-support](../../tree/master/jr-annotation-support) contains extension with support for a subset of core [Jackson annotations](../../../jackson-annotations)
* jr-all creates an "uber-jar" that contains individual modules along with all their dependencies:
    * `jr-objects` classes as-is, without relocating
    * `jr-stree` classes as-is, without relocating
    * Jackson streaming (`jackson-core`) contents *relocated* ("shaded"), for private use by `jackson-jr`
    * Does NOT contain `jr-retrofit2` or `jr-annotation-support` components

If you are not sure which package to use, the answer is usually `jr-objects`, and build system (maven, gradle) will fetch the dependency needed. `jr-all` jar is only used if the single-jar deployment (self-contained, no external dependencies) is needed.

## Status

[![Build Status](https://travis-ci.org/FasterXML/jackson-jr.svg)](https://travis-ci.org/FasterXML/jackson-jr)
[![Maven Central](https://maven-badges.herokuapp.com/maven-central/com.fasterxml.jackson.jr/jackson-jr-objects/badge.svg)](https://maven-badges.herokuapp.com/maven-central/com.fasterxml.jackson.jr/jackson-jr-objects/)
[![Javadoc](https://javadoc.io/badge/com.fasterxml.jackson.jr/jackson-jr-objects.svg)](http://www.javadoc.io/doc/com.fasterxml.jackson.jr/jackson-jr-objects)

## Usage

### Reading/writing Simple Objects, Beans, List/arrays thereof

Functionality of this package is contained in Java package `com.fasterxml.jackson.jr.ob`.

All functionality is accessed through main `JSON` Object; you can either used singleton `JSON.std`,
or construct individual objects -- either way, `JSON` instances are ALWAYS immutable and hence thread-safe.

We can start by reading JSON

```java
String INPUT = "{\"a\":[1,2,{\"b\":true},3],\"c\":3}";
Object ob = JSON.std.anyFrom(INPUT);
// or
Map<String,Object> map = JSON.std.mapFrom(INPUT);
// or
MyBean bean = JSON.std.beanFrom(MyBean.class, INPUT);
```

from any of the usual input sources (`InputStream`, `Reader`, `String` or `byte[]` that contains JSON, `URL`,
`JsonParser`); and can write same Objects as JSON:

```java
String json = JSON.std.asString(map);
JSON.std.write(ob, new File("/tmp/stuff.json");
// and with indentation; but skip writing of null properties
byte[] bytes = JSON.std
    .with(Feature.PRETTY_PRINT_OUTPUT)
    .without(Feature.WRITE_NULL_PROPERTIES)
    .asBytes(bean);
```

and may also read `List`s and arrays of simple and Bean types:

```java
List<MyType> beans = JSON.std.listOfFrom(MyType.class, INPUT);
```

(writing of `List`s and arrays works without addition effort: just pass List/array as-is)

### Reading "streaming JSON" (LD-JSON)

Version 2.10 added ability to read [Streaming JSON](https://en.wikipedia.org/wiki/JSON_streaming) content.
See ["Jackson 2.10 features"](https://medium.com/@cowtowncoder/jackson-2-10-features-cd880674d8a2) (section "Jackson-jr feature expansion") for full example, but basic
reading is done using new `ValueIterator` abstraction: 

```
File input = new File("json-stream.ldjson");
try (ValueIterator<Bean> it = JSON.std.beanSequenceFrom(Bean.class, input)) {
  while ((Bean bean = it.nextValue()) != null) {
    // do something with 'bean'
  }
}
```

### Writing with composers

An alternative method exists for writing: "fluent" style output can be used as follows:

```java
String json = JSON.std
  .with(JSON.Feature.PRETTY_PRINT_OUTPUT)
  .composeString()
  .startObject()
    .put("a", 1)
    .startArrayField("arr")
      .add(1).add(2).add(3)
    .end()
    .startObjectField("ob")
      .put("x", 3)
      .put("y", 4)
      .startArrayField("args").add("none").end()
    .end()
    .put("last", true)
  .end()
  .finish();
```

would produce (since pretty-printing is enabled)

```json
{
  "a" : 1,
  "arr" : [1,2,3],
  "ob" : {
    "x" : 3,
    "y" : 4,
    "args" : ["none"]
  },
  "last" : true
}
```

### Reading/writing JSON Trees

Jackson jr allows pluggable "tree models", and also provides one implementation, `jr-stree`.
Usage for `jr-stree` is by configuring `JSON` with codec, and then using `treeFrom` and `write` methods
like so:

```java
JSON json = JSON.std.with(new JacksonJrsTreeCodec());
TreeNode root = json.treeFrom("{\"value\" : [1, 2, 3]}");
assertTrue(root.isObject());
TreeNode array = root.get("value");
assertTrue(array.isArray());
JrsNumber n = (JrsNumber) array.get(1);
assertEquals(2, n.getValue().intValue());

String json = json.asString(root);
```

Note that `jr-stree` implementation is a small minimalistic implementation with immutable
nodes. It is most useful for simple reading use cases.

It is however possible to write your own `TreeCodec` implementations that integrate seamlessly,
and in future other tree models may be offered as part of jackson-jr, or via other libraries.

### Designing your Beans

To support readability and writability of your own types, your Java objects must either:

* Implement Bean style accessors (getters for accessing data to write and/or setter for binding JSON data into objects), and define no-argument (default) constructor, OR
* Define single-argument constructor if binding from JSON String (single-String argument) or JSON integer number (single-`long` or `Long` argument)

Note that although getters and setters need to be public (since JDK Bean Introspection does not find any other methods),
constructors may have any access right, including `private`.

Starting with version 2.8, `public` fields may also be used (although their
discovery may be disabled using `JSON.Feature.USE_FIELDS`) as an alternative:
this is useful when limiting number of otherwise useless "getter" and "setter"
methods.

NEW! Jackson-jr 2.11 introduce `jackson-jr-annotation-support` extension (see more below)
which allows use of Jackson annotations like `@JsonProperty`, `@JsonIgnore` and even `@JsonAutoDetect` for even more granular control of inclusion, naming and renaming.

### Customizing behavior with Features

There are many customizable features you can use with `JSON` object; see [Full List of Features](../../wiki/JSON-Features) for details. But usage itself is via fluent methods like so:

```java
String json = JSON.std
  .with(JSON.Feature.PRETTY_PRINT_OUTPUT)
  .without(JSON.Feature.FAIL_ON_DUPLICATE_MAP_KEYS)
  .asString(...);
```

### Adding custom value readers, writers

Version 2.10 added ability to add custom `ValueReader`s and `ValueWriter`s, to
allow pluggable support for types beyond basic JDK types and Beans.

See section "Jackson-jr ValueReaders" of [Jackson-jr 2.10 improvements](https://cowtowncoder.medium.com/jackson-2-10-jackson-jr-improvements-9eb5bb7b35f) for an explanation of how to add custom `ValueReader`s and `ValueWriter`s

You can also check out unit test

    jr-objects/src/test/java/com/fasterxml/jackson/jr/ob/impl/CustomValueReadersTest.java

for sample usage.

There is also a blog post [Enable support for java.time.* with Jackson-jr](https://www.andersaaberg.dk/2020/enable-support-for-java-time-with-jackson-jr/) which shows how to write custom readers/writers; in this
case ones for Java 8 date/time types, but the concept is general.

### Using (some of) Jackson annotations

Jackson 2.11 added a new extension (a `JacksonJrExtension`) -- `jr-annotation-support` -- that adds support for a subset of Jackson annotations.
See [jr-annotation-support/README.md](../../tree/master/jr-annotation-support) for details of this extension, but basic usage is by registering extension:

```
import com.fasterxml.jackson.jr.annotationsupport.JacksonAnnotationExtension;

JSON json = JSON.builder()
    .register(JacksonAnnotationExtension.std)
    .build();
```

and then using `JSON` instance as usual.

## Get it!

You can use Maven dependency like:

```xml
<dependency>
  <groupId>com.fasterxml.jackson.jr</groupId>
  <artifactId>jackson-jr-objects</artifactId>
  <version>2.12.0</version>
</dependency>
```

and then you can also download jars via [Central Maven repository](http://repo1.maven.org/maven2/com/fasterxml/jackson/jr/jackson-jr-objects/).

Or you can also clone the project and build it locally with `mvn clean install`.

Alternatively if you want a single jar deployment, you can use `jackson-jr-all` jar which embeds `jackson-core`
(repackaged using Shade plug-in, so as not to conflict with "vanilla" `jackson-core`):

    http://repo1.maven.org/maven2/com/fasterxml/jackson/jr/jackson-jr-all/

## Performance

Initial performance testing using [JVM Serializers](https://github.com/eishay/jvm-serializers/wiki) benchmark
suggests that it is almost as fast as [full Jackson databind](https://github.com/FasterXML/jackson-databind) --
additional overhead for tests is 5-10% for both serialization and deserialization.
So performance is practically identical.

In fact, when only handling `List`s and `Map`s style content, speed `jackson-jr` speed fully matches `jackson-databind`
performance (Bean/POJO case is where full databinding's extensive optimizations help more).
So performance should be adequate, and choice should be more based on functionality, convenience and
deployment factors.

About the only thing missing is that there is no equivalent to [Afterburner](../../../jackson-module-afterburner), which
can further speed up databind by 20-30%, for most performance-sensitive systems.
