com.cedarsoftware:json-io 4.5.0

Java JSON serialization


Platform: Maven

Language: Groovy

License: Apache-2.0

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



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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. There is a 3rd optional class (JsonObject) see 'Non-typed Usage' below.

json-io does not require that Java classes implement Serializable or Externalizable to be serialized, unlike 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.

The JsonReader / JsonWriter code does not depend on any native or 3rd party libraries.

For useful Java utilities, check out java-util at


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 options below).

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).


json-io can be used directly on JSON Strings or with Java's Streams.

Example 1: String to Java object

String json = // some JSON content
Object obj = JsonReader.jsonToJava(json);     // optional 2nd 'options' argument (see below)

This will convert the JSON String to a Java Object graph.

Example 2: Java object to JSON String

Employee emp;
// Emp fetched from database
String json = JsonWriter.objectToJson(emp);     // optional 2nd 'options' argument (see below)

This example will convert the Employee instance to a JSON String. If the JsonReader were used on this String, it would reconstitute a Java Employee instance.

Example 3: InputStream to Java object

Employee emp = (Employee) JsonReader.jsonToJava(stream);  // optional 2nd 'options' argument (see below)

In this example, an InputStream (could be from a File, the Network, etc.) is supplying an unknown amount of JSON. If you want, you can use the JsonReader to wrap the stream to parse it, and return the Java object graph it represents. See constructors that take a Stream argument.

Example 4: Java Object to OutputStream

Employee emp;
// emp obtained from database
JsonWriter jw = new JsonWriter(outputStream);       // optional 2nd 'options' argument (see below)

In this example, a Java object is written to an output stream in JSON format.

Non-typed Usage

json-io provides the choice to use the generic "Map of Maps" representation of an object, akin to a Javascript associative array. When reading from a JSON String or InputStream of JSON, the JsonReader can be constructed like this:

// shown using Groovy short-hand for Map of options.  See options below.
String json = // some JSON obtained from wherever
Object obj = JsonReader.jsonToJava(json, [(JsonReader.USE_MAPS): true])    

This will return an untyped object representation of the JSON String as a Map of Maps, where the fields are the Map keys (Strings), and the field values are the associated Map's values. In this representation the returned data consists of Maps, Arrays (Object[]), and JSON values. The Maps are actually a JsonObject instance (from json-io). This JsonObject implements the Map interface permitting access to the entire object. Cast to a JsonObject, you can see the type information, position within the JSON stream, and other information.

This 'Maps' representation can be re-written to a JSON String or Stream and the output JSON will exactly match the original input JSON stream. This permits a JVM receiving JSON strings / streams that contain class references which do not exist in the JVM that is parsing the JSON, to completely read / write the stream. Additionally, the Maps can be modified before being written, and the entire graph can be re-written in one collective write. Any object model can be read, modified, and then re-written by a JVM that does not contain any of the classes in the JSON data.

The optional values below are public constants from JsonWriter, used by placing them as keys in the arguments map.

CUSTOM_WRITER_MAP       // Set to Map<Class, JsonWriter.JsonClassWriterEx> to
                        // override the default JSON output for a given class. 
NOT_CUSTOM_WRITER_MAP   // Set to Collection<Class> to indicate classes that should
                        // not be written by a custom writer.
DATE_FORMAT             // Set this format string to control the format dates are 
                        // written. Example: "yyyy/MM/dd HH:mm".  Can also be a 
                        // DateFormat instance.  Can also be the constant 
                        // JsonWriter.ISO_DATE_FORMAT or 
                        // JsonWriter.ISO_DATE_TIME_FORMAT 
TYPE                    // Set to boolean true to force all data types to be 
                        // output, even where they could have been omitted. Set
                        // to false to prevent @type to be written. Do not set
                        // in order to minimize the number of @type's emitted.
PRETTY_PRINT            // Force nicely formatted JSON output 
                        // (See for example format)
FIELD_SPECIFIERS        // Set to a Map<Class, List<String>> which is used to 
                        // control which fields of a class are output.
FIELD_NAME_BLACK_LIST   // Set value to a Map<Class, List<String>> which will be used
                        // to control which fields on a class are not output. Black 
                        // list has always priority to FIELD_SPECIFIERS                         
ENUM_PUBLIC_ONLY        // If set, indicates that private variables of ENUMs are not 
                        // serialized.
WRITE_LONGS_AS_STRINGS  // If set, longs are written in quotes (Javascript safe).
                        // JsonReader will automatically convert Strings back
                        // to longs.  Any Number can be set from a String.
TYPE_NAME_MAP           // If set, this map will be used when writing @type values.
                        // Allows short-hand abbreviations for type names.
SHORT_META_KEYS         // If set, then @type => @t, @keys => @k, @items => @e,
                        // @ref => @r, and @id => @i
SKIP_NULL_FIELDS        // Do not write field values to output JSON when
                        // their value is null.                             
CLASSLOADER             // ClassLoader instance to use when turning String names of     
                        // classes into JVM Class instances.

The optional values below are public constants from JsonReader, used by placing them as keys in the arguments map.

CUSTOM_READER_MAP       // Set to Map<Class, JsonReader.JsonClassReaderEx> to
                        // override the default JSON reader for a given class. 
NOT_CUSTOM_READER_MAP   // Set to Collection<Class> to indicate classes that should
                        // not be read by a custom reader.
USE_MAPS                // If set to boolean true, the read-in JSON will be 
                        // turned into a Map of Maps (JsonObject) representation. Note
                        // that calling the JsonWriter on this root Map will indeed
                        // write the equivalent JSON stream as was read.
TYPE_NAME_MAP           // If set, this map will be used when writing @type values. 
                        // Allows short-hand abbreviations of type names.
UNKNOWN_TYPE            // Set to null (or leave out), unknown objects are returned
                        // as Maps.  Set to String class name, and unknown objects 
                        // will be created as with this class name, and the fields 
                        // will be set on it. Set to false, and an exception will be 
                        // thrown when an unknown object type is encountered.  The 
                        // location in the JSON will be given.
FAIL_ON_UNKNOWN_TYPE    // Set to Boolean.TRUE to have JSON reader throw JsonIoException
                        // when a @type value references a class that is not loadable
                        // from the class loader. Set to any other value (or leave out)
                        // and JSON parsing will return a Map to represent the unknown
                        // object defined by the invalid @type value.
CLASSLOADER             // ClassLoader instance to use when turning String names of     
                        // classes into JVM Class instances.


Customization technique 1: Drop unwanted fields

  • White-List support: Let's say a class that you want to serialize has a field on it that you do not want written out, like a ClassLoader reference. Use the JsonWriter.FIELD_SPECIFIERS to associate a List of String field names to a particular Class C. When the class is being written, only the fields you list will be written.

  • Black-List support: Let's say a class that you want to serialize has a field on it that you do not want written out, like a ClassLoader reference. Use the JsonWriter.FIELD_NAME_BLACK_LIST to associate a List of String field names to a particular Class C. When the class is being written, any field listed here will not be written. Black-listed fields take priority over white listed fields.

Customization technique 2: Custom instantiator JsonReader.assignInstantiator(Class c, ClassFactoryEx)

There are times when json-io cannot instantiate a particular class even though it makes many attempts to instantiate a class, including looping through all the constructors (public, private) and invoking them with default values, etc.
However, sometimes a class just cannot be constructed, for example, one that has a constructor that throws an exception if particular parameters are not passed into it.

In these instances, use the JsonReader.assignInstantiator(class, Factory) to assign a ClassFactory or ClassFactoryEx that you implement to instantiate the class. json-io will call your ClassFactory.newInstance(Class c) (or ClassFactoryEx.newInstance(Class c, Map args)) to create the class that it could not construct. Your ClassFactory will be called to create the instance. In case you need values from the object being instantiated in order to construct it, use the ClassFactoryEx to instantiate it. This class factory has the API newInstance(Class c, Map args) which will be called with the Class to instantiate and the JSON object that represents it (already read in). In the args Map, the key 'jsonObj' will have the associated JsonObject (Map) that is currently being read. You can pull field values from this object to create and return the instance. After your code creates the instance, json-io will reflectively stuff the values from the jsonObj (JsonObject) into the instance you create.

Customization technique 3: Shorter meta-keys (@type -> @t, @id -> @i, @ref -> @r, @keys -> @k, @items -> @e)

Set JsonWriter.SHORT_META_KEYS to true to see the single-letter meta keys used in the outputted JSON. In addition to the shorter meta keys, you can and a list of substitutions of your own to use. For example, you may want to see alist instead of java.util.ArrayList. This is only applicable if you are writing with @types in the JSON.

  Map args = [
  String json = JsonWriter.objectToJson(list, args)

In this example, we create an 'args' Map, set the key JsonWriter.SHORT_META_KEYS to true and set the JsonWriter.TYPE_NAME_MAP to a Map that will be used to substitute class names for short-hand names.

Customization technique 4: Custom serializer

New APIs have been added to allow you to associate a custom reader / writer class to a particular class if you want it to be read / written specially in the JSON output. The json-io approach allows you to customize the JSON format for classes for which you do not have the source code.

Example (in Groovy). Note the Person has a List of pets, and in this case, it re-uses 
JsonWriter to write that part of the class out (no need to customize it):

static class CustomPersonWriter implements JsonWriter.JsonClassWriterEx
    void write(Object o, boolean showType, Writer output, Map<String, Object> args) throws IOException
        Person p = (Person) o
        JsonWriter writer = Support.getWriter(args)
        writer.writeImpl(p.getPets(), true)

Customization technique 5: Processing JSON from external sources.

When reading JSON from external sources, you may want to start with:

in Groovy:

Object data = JsonReader.jsonToJava(json, [(JsonReader.USE_MAPS): true])

In Java:

Map args = new HashMap();
args.put(JsonReader.USE_MAPS, true);
Object data = JsonReader.jsonToJava(json, args);

This will get the JSON read into memory, in a Map-of-Maps format, similar to how JSON is read into memory in Javascript. This will get you going right away.

To write 'generic' JSON (without @type or @items, etc.) entries, use:

in Groovy:

String json = JsonWriter.objectToJson(objToWrite, [(JsonWriter.TYPE):false])

In Java:

Map args = new HashMap();
args.put(JsonWriter.TYPE, false);
String json = JsonWriter.objectToJson(objToWrite, args);

Objects will not include the @type flags or @items. This JSON passes nicely to non-Java receivers, like Javascript. Keep in mind, you will be working with the JSON as generic object.field and object[index] with this approach.

Please note that if you write your object graph out with JsonWriter.TYPE: false, the shape of the graph will be maintained. What this means, is that if two different places in your object graph point to the same object, the first reference will write the actual object, the 2nd and later references will write a reference (@ref) to the first instance. This will read in just fine with JsonReader.jsonToJava(), and the appropriate Map reference will be placed in all referenced locations. If reading this in Javascript, make sure to use the included jsonUtil.js to parse the read in JSON so that it can perform the substitutions of the @ref's. (See src/test/resource folder for jsonUtil.js).


Included is a small Javascript utility (jsonUtil.js in the src/test/resources folder) that will take a JSON output stream created by the JSON writer and substitute all @ref's for the actual pointed to object. It's a one-line call - resolveRefs(json). This will substitute @ref tags in the JSON for the actual pointed-to object.

What's next?

Even though json-io is perfect for Java / Javascript serialization, there are other great uses for it:


Many projects use JsonWriter to write an object to JSON, then use the JsonReader to read it in, perfectly cloning the original object graph:

Employee emp;
// emp obtained from database
Employee deepCopy = (Employee) cloneObject(emp);

public Object cloneObject(Object root)
    return JsonReader.jsonToJava(JsonWriter.objectToJson(root));


Instead of doing System.out.println() debugging, call JsonWriter.objectToJson(obj) and dump that String out. It will reveal the object in all it's glory.

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. Used this way, you can create more active sites like Google's GMail, MyOtherDrive online backup, etc.

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

Featured on

See for revision history.

by John DeRegnaucourt


  • 4.5.0 - July 17, 2016 01:18
  • 4.4.0 - March 09, 2016 12:26
  • 4.3.1 - March 03, 2016 05:25
  • 4.3.0 - December 09, 2015 06:59
  • 4.2.1 - December 05, 2015 15:34
  • 4.2.0 - December 03, 2015 14:16
  • 4.1.10 - December 02, 2015 05:34
  • 4.1.9 - October 28, 2015 03:35
  • 4.1.8 - October 18, 2015 02:37
  • 4.1.7 - October 18, 2015 01:56
See all 66 releases

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