DSL-JSON library

DSL Platform compatible JSON library for Java and Android.

Java JSON library designed for performance. Built for invasive software composition with DSL Platform compiler.

Distinguishing features

supports external schema - Domain Specification Language (DSL)
works on existing POJO classes via annotation processor - it converts POJO to DSL schema and constructs specialized converters at compile time
performance - faster than any other Java JSON library. On pair with fastest binary JVM codecs
works on byte level - deserialization can work on byte[] or InputStream. It doesn't need intermediate char representation
extensibility - custom types can be registered for serialization/deserialization
streaming support - large JSON lists support streaming with minimal memory usage
zero-copy operations - converters avoid producing garbage
minimal size - runtime dependency weights around 100KB
no runtime overhead - both schema and annotation based POJOs are prepared at compile time
no unsafe code - library doesn't rely on Java UNSAFE/internal methods
legacy name mapping - multiple versions of JSON property names can be mapped into a single POJO using alternativeNames annotation

Schema based serialization

DSL can be used for defining schema from which POJO classes with embedded JSON conversion are constructed. This is useful in large, multi-language projects where model is defined outside of Java classes. More information about DSL can be found on DSL Platform website.

@CompiledJson annotation

Annotation processor works by translating Java classes into DSL and running DSL Platform compiler on it. DSL compiler will generate optimized converters and register them into META-INF/services. This will be loaded during DslJson initialization with ServiceLoader. Converters will be created even for dependent objects which don't have @CompiledJson annotation. This can be used to create serializers for pre-existing classes without annotating them. Both bean properties and public non-final fields are supported.

Annotation processor can be added as Maven dependency with:

<dependency>
  <groupId>com.dslplatform</groupId>
  <artifactId>dsl-json-processor</artifactId>
  <version>1.4.1</version>
  <scope>provided</scope>
</dependency>

For use in Android, Gradle can be configured with:

apply plugin: 'android-apt'
dependencies {
  compile compile 'com.dslplatform:dsl-json:1.3.1'
  apt 'com.dslplatform:dsl-json-processor:1.4.1'
}

Project examples can be found in examples folder

Java/DSL property mapping

Java type	DSL type	Java type	DSL type
int	int	byte[]	binary
long	long	java.util.Map<String,String>	properties?
float	float	java.net.InetAddress	ip?
double	double	java.awt.Color	color?
boolean	bool	java.awt.geom.Rectangle2D	rectangle?
java.lang.String	string?	java.awt.geom.Point2D	location?
java.lang.Integer	int?	java.awt.geom.Point	point?
java.lang.Long	long?	java.awt.image.BufferedImage	image?
java.lang.Float	float?	android.graphics.Rect	rectangle?
java.lang.Double	double?	android.graphics.PointF	location?
java.lang.Boolean	bool?	android.graphics.Point	point?
java.math.BigDecimal	decimal?	android.graphics.Bitmap	image?
java.time.LocalDate	date?	org.w3c.dom.Element	xml?
java.time.OffsetDateTime	timestamp?	org.joda.time.LocalDate	date?
java.util.UUID	uuid?	org.joda.time.DateTime	timestamp?

Java/DSL collection mapping

Java type	DSL type
array	Array
java.util.List	List
java.util.Set	Set
java.util.LinkedList	Linked List
java.util.Queue	Queue
java.util.Stack	Stack
java.util.Vector	Vector
java.util.Collection	Bag

Collections can be used on supported Java types, other POJOs and enums.

Custom types

Types without builtin mapping can be supported in three ways:

by implementing JsonObject and appropriate JSON_READER
by defining custom conversion class and annotating it with @JsonConverter
by defining custom conversion class and referencing it from property with converter through @JsonAttribute

Custom converter for java.util.Date can be found in example project Annotation processor will check if custom type implementations have appropriate signatures. Converter for java.util.ArrayList can be found in same example project

@JsonAttribute features

DSL-JSON property annotation supports several customizations/features:

name - define custom serialization name
alternativeNames - different incoming JSON attributes can be mapped into appropriate property. This can be used for simple features such as casing or for complex features such as model evolution
ignore - don't serialize specific property into JSON
nullable - tell compiler that this property can't be null. Compiler can remove some checks in that case for minuscule performance boost
hashMatch - DSL-JSON matches properties by hash values. If this option is turned off exact comparison will be performed which will add minuscule deserialization overhead, but invalid properties with same hash names will not be deserialized into "wrong" property. In case when model contains multiple properties with same hash values, compiler will inject exact comparison by default, regardless of this option value.
converter - custom conversion per property. Can be used for formatting or any other custom handling of JSON processing for specific property

External annotations

For existing classes which can't be modified with @JsonAttribute alternative external annotations are supported:

Nullability annotations

During translation from Java objects into DSL schema, existing type system nullability rules are followed. With the help of non-null annotations, hints can be introduced to work around some Java nullability type system limitations. List of supported non-null annotations can be found in processor source code

Property aliases

Annotation processor supports external annotations for customizing property name in JSON:

com.fasterxml.jackson.annotation.JsonProperty
com.google.gson.annotations.SerializedName

Those annotations will be translated into specialized DSL for specifying serialization name.

Ignored properties

Existing bean properties and fields can be ignored using one of the supported annotations:

com.fasterxml.jackson.annotation.JsonIgnore
org.codehaus.jackson.annotate.JsonIgnore

Ignored properties will not be translated into DSL schema.

Serialization modes

Library has two serialization modes:

minimal serialization - omits default properties which can be reconstructed from schema definition
all properties serialization - will serialize all properties from schema definition

Best serialization performance can be obtained with combination of minimal serialization and minified property names/aliases.

Benchmarks

Independent benchmarks can validate the performance of DSL-JSON library:

JVM serializers - benchmark for all kind of JVM codecs. Shows DSL-JSON as fast as top binary codecs
Techempower round 13 - servlet equiped with DSL-JSON tops the list
Kostya JSON - fastest performing Java JSON library
JMH JSON benchmark - benchmarks for Java JSON libraries

Reference benchmark (built by library authors):

.NET vs JVM JSON - comparison of various JSON libraries

Dependencies

To create compile time databinding, annotation processor will invoke DSL compiler, which requires Mono/.NET. There is no runtime Mono/.NET dependency, only JVM. Java8 Java-Time API is supported as a separate jar, since core library targets Java6.

Library can be added as Maven dependency with:

<dependency>
  <groupId>com.dslplatform</groupId>
  <artifactId>dsl-json</artifactId>
  <version>1.3.1</version>
</dependency>

Best practices

Reusing reader/writer.

JsonWriter should be reused since it contains growable byte[] buffer for encoding objects into JSON. For thread reuse use something like ThreadLocal<JsonWriter>. After serialization copy resulting buffer to stream with .toStream(OutputStream) method.

JsonReader works on byte[] input. It's best to construct JsonReader with reusable byte[] and specifying int length. For InputStream JsonStreamReader can be used. For small messages it's better to use byte based reader instead of stream based reader.

FAQ

Q: What is TContext in DslJson and what should I use for it?
A: Generic TContext is used for library specialization. Use DslJson<Object> when you don't need it and just provide null for it.

Q: Why is DSL-JSON faster than others?
A: Almost zero allocations. Works on byte level. Better algorithms for conversion from byte[] -> type and vice-versa. Minimized unexpected branching.

Q: DslJson is failing with unable to resolve reader/writer. What does it mean?
A: During startup DslJson loads services through ServiceLoader. For this to work META-INF/services/com.dslplatform.json.Configuration must exist with the content of dsl_json.json.ExternalSerialization which is the class crated during compilation step. Make sure you've referenced processor library (which is responsible for setting up readers/writers during compilation) and double check if annotation processor is running. Refer to example projects for how to set up environment.

Q: Maven/Gradle are failing during compilation with @CompiledJson. What can I do about it?
A: If Mono/.NET is available it should work out-of-the-box. But if some strange issue occurs, detailed log can be enabled to see what is causing the issue. Log is disabled by default, since some Gradle setups fail if something is logged during compilation. Log can be enabled with dsljson.loglevel processor option

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