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

<dependency>
  <groupId>com.cedarsoftware</groupId>
  <artifactId>json-io</artifactId>
  <version>4.1.9</version>
</dependency>

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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 and powerful Java utilities, check out java-util at http://github.com/jdereg/java-util

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.

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

Object obj = JsonReader.jsonToJava("[\"Hello, World\"]");

This will convert the JSON String to a Java Object graph. In this case, it would consist of an Object[] of one String element.

Example 2: Java object to JSON String

Employee emp;
// Emp fetched from database
String json = JsonWriter.objectToJson(emp);

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

JsonReader jr = new JsonReader(inputStream);
Employee emp = (Employee) jr.readObject();

In this example, an InputStream (could be from a File, the Network, etc.) is supplying an unknown amount of JSON. The JsonReader is used to wrap the stream to parse it, and return the Java object graph it represents.

Example 4: Java Object to OutputStream

Employee emp;
// emp obtained from database
JsonWriter jw = new JsonWriter(outputStream);
jw.write(emp);
jw.close();

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

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:

Map graph = JsonReader.jsonToMaps(String json);

-- or --

JsonReader jr = new JsonReader(InputStream, true);
Map map = (Map) jr.readObject();

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 Map instance returned is 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!

Both the JsonWriter and JsonReader allow you to pass in an optional arguments Map<String, Object>. This Map has well known keys (constants from JsonWriter / JsonWriter). To enable the respective feature, first create a Map. Then place the well known key in the Map and associate the appropriate setting as the value. Below is a complete list of features and some example usages. Shown in Groovy for brevity.

Map args = [
        (JsonWriter.SHORT_META_KEYS):true,
        (JsonWriter.TYPE_NAME_MAP):[
            'java.util.ArrayList':'alist', 
            'java.util.LinkedHashMap':'lmap', 
            (TestObject.class.getName()):'testO'
        ]
]
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.

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 http://jsoneditoronline.org for example format)
FIELD_SPECIFIERS        // Set to a Map<Class, List<String>> which is used to 
                        // control which fields of a class are output. 
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 

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.

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. json-io 1.x required a custom method be implemented on the object which was having its JSON format customized. This support has been removed. That approach required access to the source code for the class being customized. The new json-io 2.0+ 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 (not 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
        output.write('"first":"')
        output.write(p.getFirstName())
        output.write('","last":"')
        output.write(p.getLastName())
        JsonWriter writer = Support.getWriter(args)
        writer.writeImpl(p.getPets(), true)
    }
}

Included is a small Javascript utility 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. In addition, the @keys / @items will also be converted into Javascript Maps and Arrays. Finally, there is a Javascript API that will convert a full Javascript object graph to JSON, (even if it has cycles within the graph). This will maintain the proper graph-shape when sending it from the client back to the server.

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.

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.

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 https://github.com/jdereg/json-command-servlet for a light-weight servlet that processes Ajax / XHR calls.

Featured on http://json.org.

YourKit supports open source projects with its full-featured Java Profiler. YourKit, LLC is the creator of YourKit Java Profiler and YourKit .NET Profiler, innovative and intelligent tools for profiling Java and .NET applications.

• 4.1.9
• Bug fix: When writing a Map that has all String keys, the keys were not being escaped for quotes (UTF-8 characters in general).
• 4.1.8
• Bug fix: 4.1.7 skipped ALL transient fields. If a transient field is listed in the field specifiers map, then it must be traced.
• 4.1.7
• Bug fix: Transient fields are skipped during reference tracing. (fix submitted by Francis Upton, @francisu). Some transient fields could cause an exception to be thrown when being trace for references, stopping serialization.
• 4.1.6
• Better support for primitive output when 'never show type' is set. (submitted by @KaiHufenbach)
• 4.1.5
• Tests updated to use Groovy 2.4.4
• Deserialization updated to handle objects where the referencing class uses an Object pointer and writes the value out as single primitive value, using the 'value' key. (submitted by @KaiHufenbach)
• pom filed updated to use a maven bundle plugin (Apache Felix) to generate OSGI headers (submitted by @KaiHufenbach)
• 4.1.4
• Bug fix: Custom readers will now always have the .target field set if a JsonObject is passed to them. The custom reader's read() method was being called before the .target field was set on the JsonObject.
• 4.1.3
• Made JsonReader / JsonWriter getObjectsReferenced() API public (allows custom reader / writers access to these)
• Resolver.createJavaObjectInstance(), used to create the correct Java object for a JsonObject peer, no longer calls the .read() API for objects's with custom readers.
• 4.1.2
• All objects in the graph are 'traced' (JsonWriter.traceReferences) except references. The code used to not trace fields on objects that were handled by custom writers.
• 4.1.1
• JDK 1.6 support - Use of ReflectiveOperationException changed to InvocationTargetException.
• 4.1.0
• JDK 1.6 support restored. Keeping 1.6 support for Android developers. Submitted by @kkalisz
• 4.0.1
• To prevent @type from being written, set the optional argument JsonWriter.TYPE = false. This is generally not recommended, as the output JSON may not be able to be re-read into Java objects. However, if the JSON is destined for a non-Java system, this can be useful.
• 4.0.0
• Custom readers / writers are set now per-instance of JsonReader / JsonWriter, not static. This allows using different customization for cloning, for example, than for serialization to client.
• JsonReader.jsonToJava() and JsonReader.jsonToMaps() now allow an InputStream to be used.
• Custom readers / writers can now be set all-at-once through the optional 'args' Map.
• 'notCustom' readers / writers can now be set all-at-once through the optional 'args' Map.
• The removeReader(), removeWriter(), removeNotCustomReader(), and removeNotCustomWriter() APIs have been removed since customizers are set per-instance.
• 3.3.2
• Added new JsonObject.isReference() API which will return 'true' if the JsonObject is currently representing a reference @ref
• Added new JsonReader.getRefTarget(jsonObject) API which will follow the @ref links until it resolves to the referenced (target) instance.
• Added new JsonReader() constructor that only takes the args (Map). It is expected that you will call JsonReader.jsonObjectsToJava(rootJsonObject) which will parse the passed in JsonObject graph.
• Added new JsonReader.removeReader() API to remove a custom reader association to a given class.
• Added new JsonWriter.removeWriter() API to remove a custom writer association to a given class.
• Added new JsonReader.removeNotCustomReader() API to remove a not custom reader - if a notCustom() reader has been added (preventing inherited object from using custom reader), the association can be eliminated using this API.
• Added new JsonWriter.removeNotCustomWriter() API to remove a not custom writer - if a notCustom() writer has been added (preventing inherited object from using custom writer), the association can be eliminated using this API.
• 3.3.1
• Re-entrancy issue fixed. If a CustomReader (or CustomWriter) instantiated another copy of JsonReader or JsonWriter (indirectly, through recursion, for example), the 2nd instance of JsonReader or JsonWriter would clobber the ThreadLocal values inside JsonReader / JsonWriter. Those ThreadLocal values have been removed and converted to per-instance member variables.
• 3.3.0
• Consolidate all 3.2.x changes
• Last snippet read no longer shows 'boxes' for unused internal buffer characters.
• JsonWriter - moved reference check 'up' to writeImpl() so that each specific 'write' routine did not have to test / call writeOptionalReference().
• If you have a custom reader that does not bother to resolve references from 'deeper' internal JsonObject maps, an exception will no longer be thrown. It is OK for a custom reader not to 'care' about internal deeper fields if it wants to ignore them.
• 3.2.3
• Cache Map's for custom reader's updated to be ConcurrentMap instead of Map.
• 3.2.2
• JsonCustomReaderEx added, which passes the 'args' Map through to the custom reader.
• Both JsonCustomReaderEx and JsonCustomWriterEx have a Map as the last argument in their single method that is implemented by the custom reader / writer. This Map is the same as the 'args' Ma passed into to the JsonReader / JsonWriter, with the added JSON_READER or JSON_WRITER key and associated value of the calling JsonReader / JsonWriter instance.
• 3.2.1
• Made Support.getWriter() method public static so that CustomWriters can easily use it
• Changed JsonCustomWriterEx to no longer inherit from JsonCustomWriter and instead added a common parent (JsonCustomWriterBase). This allows only one method to be overridden to create a JsonCustomWriterEx.
• 3.2.0
• New JsonCustomWriterEx interface which adds the JsonWriter access to the implementing class so that it can call back and use jsonWriter.writeImpl() API.
• Change JsonWriter.writeImpl() from protected to public
• 3.1.3
• Performance improvement: No longer using .classForName() inside JsonObject to determine isMap() or isCollection(). Reading JSON into Map of Maps mode significantly faster.
• 3.1.2
• Bug fix: Version 3.1.1 introduced a bug where it would always run as though it was in JSON to Java mode always (as opposed to supporting JSON to Maps). This has been fixed.
• 3.1.1
• JsonReader.UNKNOWN_OBJECT added as an option to indicate what to do when an unknown object is encountered in the JSON. Default is a Map will be created. However, you can set this argument to a String class name to instantiate, or set it to false to force an exception to be thrown.
• 3.1.0
• New Feature: Short class names to reduce the size of the output JSON. This allows you to, for example, substitute java.util.HashMap with hmap so that it will appear in the JSON as "@type":"hmap". Pass the substitution map to the JsonWriter (or reader) as an entry in the args Map with the key of JsonWriter.TYPE_NAME_MAP and the value as a Map instance with String class names as the keys and short-names as the values. The same map can be passed to the JsonReader and it will properly read the substituted types.
• New Feature: Short meta-key names to reduce the size of the output JSON. The @type key name will be shortened to @t, @id => @i, @ref => @r, @keys => @k, @items => @e. Put a key in the args Map as JsonWriter.SHORT_META_KEYS with the value true.
• 3.0.2
• Bug fix: Using a CustomReader in a Collection with at least two identical elements causes an exception (submitted by @KaiHufenbach).
• 3.0.1
• Added new flag JsonWriter.WRITE_LONGS_AS_STRINGS which forces long/Long's to be written as Strings. When sending JSON data to a Javascript, longs can lose precision because Javascript only maintains 53-bits of info (Javascript uses IEEE 754 double for numbers). The precision is lost due to some of the bits used for maintaining an exponent. With this flag set, longs will be sent as Strings, however, on return back to a Java server, json-io allows Strings to be set right back into long (fields, array elements, collections, etc.)
• 3.0.0
  • Performance improvement: caching the custom readers and writes associated to given classes.
  • Ease of use: json-io throws a JsonIoException (unchecked) instead of checked exception IOException. This allows more flexibility in terms of error handling for the user.
  • Code cleanup: Moved reflection related code from JsonReader into separate MetaUtils class.
  • Code cleanup: Moved FastPushbackReader from JsonReader into separate class.
  • Code cleanup: Moved JSON parsing code from JsonReader into separate JsonParser class.
  • Code cleanup: Moved built-in readers from JsonReader to separate Readers class.
  • Code cleanup: Moved resolver code (marshals map of maps to Java instances) into separate Resolver classes.
• 2.9.4
• JsonReader.newInstance() API made public
• Bumped version of junit from 4.11 to 4.12
• Added additional tests to ensure that null and "" can be properly assigned to primitive values (matching behavior of java-util's Converter.convert() API).
• 2.9.3
• Bug fix: When writing a Map with JSON primitive keys (String, Long, Double, or Boolean), a ClassCastException was being thrown if the type was Long, Double, or Boolean. This has been fixed with test added.
• 2.9.2
• Android: Rearranged [:.] to [.:] in regular expressions for Android compatibility. Technically, it should not matter, but [:.] was causing java.util.regex.PatternSyntaxException: Syntax error U_ILLEGAL_ARGUMENT_ERROR on Android JVM.
• Bug fix: When using the JsonWriter arguments Map with FIELD_SPECIFIERS, if you specified a field that was transient, it was not serialized. This has been corrected. When you specify the field list for a given class, the Map can contain any non-static fields in the class, including transient fields.
• All JUnit tests converted to Groovy.
• 2.9.1
• Bug fix: Parameterized types are only internally stamped onto generic Maps (Maps read with no @type) if the field that points to the Map is a template variable or it has template arguments.
• Performance optimization: tracing references specially handles Collection and Map. By avoiding internal structures, the reference trace is much faster.
• 2.9.0
• Unmodifiable Collections and Maps can now be serialized.
• Added tests to ensure that JsonReader.jsonToMaps() coerces the RHS values when logical primitives, to the optional associated @type's fields.
• More tests and improved code-coverage.
• 2.8.1
• Bug fix: JsonReader.jsonToMaps() API was incorrectly attempting to instantiate peer objects (specified by "@type" field in the JSON) when in 'maps' mode. This made JsonReader.jsonToMaps() fail if all referenced class names did not exist in the JVM. This has been fixed.
• Minor Javadoc cleanup (Daniel Darabos @darabos)
• Began migration of tests from one monolithic Java class (TestJsonReaderWriter) to individual Groovy test classes.
• 2.8.0
• Additional attempt to instantiate classes via sun.misc.Unsafe added (optional must be turned on by calling JsonReader.setUseUnsafe(true)). json-io already tries all constructors (private or public) with varying arguments, etc. If this fails and unsafe is true, it will try sun.misc.Unsafe.allocateInstance() which effectively does a C-style malloc(). This is OK, because the rest of JsonReader fills in the member variables from the serialized content. (Submitted by @KaiHufenbach).
• 2.7.6
• Performance optimizations. Use of switch statement instead of if-else chains.
• JDK 1.7 for source code and target JVM.
• 2.7.5
• Bug fix: ArrayIndexOutOfBounds could still occur when serializing a class with multiple Templated fields. The exception has been fixed.
• 2.7.4
• Bug fix: ArrayIndexOutOfBounds exception occurring when serializing non-static inner class with nested template parameters. JsonReader was incorrectly passing on the 'this$0' field for further template argument processing when it should not have.
• 2.7.3
• JsonReader executes faster (more efficiently manages internal 'snippet' buffer and last line and column read.)
• Improved date parsing: day of week support (long or short name), days with suffix (3rd, 25th, etc.), Java's default .toString() output for Date now parses, full time zone support, extra whitespace allowed within the date string.
• Added ability to have custom JSON writers for interfaces (submitted by @KaiHufenbach).
• 2.7.2
• When writing JSON, less memory is used to manage referenced objects. JsonWriter requires a smaller memory foot print during writing.
• New option available to JsonWriter that allows you to force enums to not write private variables. First you can make them transient. However, if you do not own the code or cannot change it, you can set the JsonWriter.getArgs().put(ENUM_PUBLIC_ONLY, true), and then only public fields on enums will be emitted.
• 2.7.1
• BigDecimal and BigInteger are now always written as a primitive (immutable, non-referenced) value. This uniformizes their output.
• 2.7.0
• Updated to support JSON root of String, Integer, Floating point, and Boolean, per the updated JSON RFP. Example, the String "football" is considered valid JSON. The JsonReader.readObject() API and JsonReader.jsonToJava() will return a String in this case. The JsonReader.jsonToMaps() API will still return a Map (JsonObject), and the @items key will contain an Object[] with the single value (String, Integer, Double, Boolean) in it.
• When a Java Map has only String keys in it, json-io will use the JSON object keys directly and associate the values to the keys as expected. For example, the Map ['Football':true] would be written {"Football":true}. However, if the keys are non-Strings, then Maps will be written as a JSON object with {"@keys":[...], "@items":[...]}, where @keys is an array [] of all the keys, and the @items is an array [] of all the values. Entry 0 of @keys matches with Entry 0 in the @items array, and so on. Thanks for Christian Reuschling for making the request and then supplying the implementation.
• Change some APIs from private to protected to allow for subclasses to more easily override the default behavior.
• 2.6.1
• Bug fix: An internal Map that kept meta-information about a Java Class, changed to ConcurrentHashMap from HashMap.
• 2.6.0
• Added support for specifying which fields on a class will be serialized. Use the JsonWriter.FIELD_SPECIFIERS key and assign the value to a Map<Class, List<String>>, where the keys of the Map are classes (e.g. Bingo.class) and the values are List<String>, which indicates the fields to serialize for the class. This provides a way to reduce the number of fields written for a given class. For example, you may encounter a 3rd Party class which fails to serialize because it has an oddball field like a ClassLoader reference as a non-static, non-transient field. You may not have access to the source code to mark the field as transient. In this case, add the appropriate entries in the FIELD_SPECIFIERS map. Voila, problem solved. Use the JsonWriter API that takes optionalArgs Map. The key for this Map is JsonWriter.FIELD_SPECIFIER and the value is Map<Class, List<String>>.
• 2.5.2
• java.net.URL can now be used as a constructor argument. The reader was throwing an exception instantiating a constructor with a URL parameter.
• java.lang.Object parameters in constructor arguments are now tried with both null and new Object() now.
• 2.5.1
• Fixed a bug (introduced in 2.5.0) in the processing of a Map that has a Collection as a key.
• 2.5.0
• New 'Pretty-Print' option available. If the 'args' Map passed to JsonWriter.objectToJson(o, args) contains the key JsonWriter.PRETTY_PRINT and the value 'true' (boolean or String), the JsonWriter output will be formatted in a nice human readable format.
• Convert a JSON String to Pretty-Print format using JsonWriter.formatJson(String json). A String will be returned with the JSON formatted in a nice, human readable format.
• If a Field contains Parameterized types (e.g., Map<String, Set<Long>>, and so on), JsonReader will use those fields to process objects deep within Maps, Collections, etc. and still create the proper Java class.
• 2.4.5
• Allow "" to be set into Date field, setting the Date field (or Date array element) as null.
• 2.4.4
• Allow "" to be set into BigInteger or BigDecimal when return value is Map (JsonObject). "" to non-String fields will be null, except for primitives and primitive wrappers, that will result in JVM default value.
• 2.4.2
• Allow "" to be set into non-String fields, when doing so, null is set on Object type fields; on primitive fields, the JVM default value is set. This is for when converting JSON to Java objects directly.
• 2.4.1
• Added support to allow primitives and String to be assigned to abstract / interface / base type field on an object (Serializable, Comparable, Object, etc.). Primitives can now be 'set' into these fields, without any additional type information.
• 2.4.0
• Primitives can be set from Strings
• Strings can be set from primitives
• BigDecimal and BigInteger can be set from primitives, Strings, BigDecimal, or BigInteger
• 2.3.0
• Maps and Collections (Lists, Set, etc.) can be read in, even when there are no @keys or @items as would come from a Javascript client.
• json-io will now use the generic info on a Map<Foo, Bar> or Collection<Foo> object's field when the @type information is not included. json-io will then know to create Foo instances, Bar instances, etc. within the Collection or Map.
• All parsing error messages now output the last 100 characters read, making it easier to locate the problem in JSON text. Furthermore, line and column number are now included (before it was a single position number). This allows you to immediately find the offending location.
• You can now force @type to be written (not recommended) by putting the JsonWriter.TYPE key in the JsonWriter args map, and assigning the associated value to true.
• 2.2.32
• Date/Time format can be customized when writing JSON output. New optional Map args parameter added to main API of JsonWriter that specifies additional parameters for JsonWriter. Set the key to JsonWriter.DATE_FORMAT and the value to a SimpleDateFormat string. Two ISO formats are available for convenience as constants on JsonWriter, JsonWriter.ISO_DATE_FORMAT and JsonWriter.ISO_DATE_TIME_FORMAT.
• JsonReader updated to read many different date/time formats.
• When JsonReader encounters a class that cannot be constructed, you can associate a ClassFactory to the class, so that then the un-instantiable class is encountered, your factory class will be called to create the class. New API: JsonReader.assignInstantiator(Class c, ClassFactory factory)
• 2.2.31
• Adds ability to instantiate a wider range of constructors. This was done by attempting construction with both null and non-null values for many common class types (Collections, String, Date, Timezone, etc.)
• 2.2.30
• java.sql.Date when read in, was instantiated as a java.util.Date. This has been corrected.
• 2.2.29
• First official release through Maven Central

by John DeRegnaucourt