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https://github.com/FasterXML/jackson-jr

Stand-alone data-binding module designed as a light-weight (and -featured) alternative to `jackson-databind`: will only deal with "Maps, Lists, Strings, wrappers and Java Beans" (jr-objects), or simple read-only trees (jr-stree)
https://github.com/FasterXML/jackson-jr

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Stand-alone data-binding module designed as a light-weight (and -featured) alternative to `jackson-databind`: will only deal with "Maps, Lists, Strings, wrappers and Java Beans" (jr-objects), or simple read-only trees (jr-stree)

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README 

        
# 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 starting with 2.11 there is even optional support for a subset of Jackson annotations

via optional `jackson-jr-annotation-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](https://proguard.sourceforge.net/) can bring the jar

size down even further, by renaming and removing debug information.



## License



Good old [Apache License](https://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](https://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-extension-javatime](../../tree/master/jr-extension-javatime) contains extension with support for a subset of Java 8 Date/Time types (e.g. `java.time.LocalDateTime`)

* 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)](https://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 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 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 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



  com.fasterxml.jackson.jr

  jackson-jr-objects

  2.16.1



```



and then you can also download jars via [Central Maven repository](https://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`):



    https://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.