Perfect Java serialization to and from JSON format (available on Maven Central). To include in your project:

Programming language: Java
License: Apache License 2.0
Tags: JSON    
Latest version: v4.12.0

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Perfect Java serialization to and from JSON format (available on Maven Central). To include in your project:


json-io consists of two main classes, a reader (JsonReader) and a writer (JsonWriter). json-io eliminates the need for using ObjectInputStream / ObjectOutputStream to serialize Java and instead uses the JSON format.

json-io does not require that Java classes implement Serializable or Externalizable to be serialized, unlike the JDK's ObjectInputStream / ObjectOutputStream. It will serialize any Java object graph into JSON and retain complete graph semantics / shape and object types. This includes supporting private fields, private inner classes (static or non-static), of any depth. It also includes handling cyclic references. Objects do not need to have public constructors to be serialized. The output JSON will not include transient fields, identical to the ObjectOutputStream behavior.

json-io does not depend on any 3rd party libraries, has extensive support for Java Generics, and allows extensive customization.

A few advantages of json-io over Google's gson library:

  • gson will fail with infinite recursion (StackOverflowError) when there is a cycle in the input data. Illustrated here.
  • gson cannot handle non-static inner classes. Illustrated here.
  • gson cannot handle hetereogeneous Collections, Object[], or Maps. Illustrated here.
  • gson cannot handle Maps with keys that are not Strings. Illustrated here.


json-io uses proper JSON format. As little type information is included in the JSON format to keep it compact as possible. When an object's class can be inferred from a field type or array type, the object's type information is left out of the stream. For example, a String[] looks like ["abc", "xyz"].

When an object's type must be emitted, it is emitted as a meta-object field "@type":"package.class" in the object.
When read, this tells the JsonReader what class to instantiate. (@type output can be turned off - see User Guide).

If an object is referenced more than once, or references an object that has not yet been defined, (say A points to B, and B points to C, and C points to A), it emits a "@ref":n where 'n' is the object's integer identity (with a corresponding meta entry "@id":n defined on the referenced object). Only referenced objects have IDs in the JSON output, reducing the JSON String length.


json-io was written with performance in mind. In most cases json-io is faster than the JDK's ObjectInputStream / ObjectOutputStream. As the tests run, a log is written of the time it takes to serialize / deserialize and compares it to ObjectInputStream / ObjectOutputStream (if the static variable _debug is true in TestUtil).

User Guide

Pretty-Printing JSON

Use JsonWriter.formatJson() API to format a passed in JSON string to a nice, human readable format. Also, when writing JSON data, use the JsonWriter.objectToJson(o, args) API, where args is a Map with a key of JsonWriter.PRETTY_PRINT and a value of 'true' (boolean or String). When run this way, the JSON written by the JsonWriter will be formatted in a nice, human readable format.

RESTful support

json-io can be used as the fundamental data transfer method between a Javascript / JQuery / Ajax client and a web server in a RESTful fashion.

See json-command-servlet for a light-weight servlet that processes REST requests.


For useful Java utilities, check out java-util

Featured on json.org.

Revision History


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Copyright (c) 2007 Cedar Software LLC.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at


Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
See the License for the specific language governing permissions and
limitations under the License.

by John DeRegnaucourt

*Note that all licence references and agreements mentioned in the JSON-io README section above are relevant to that project's source code only.