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
• binding to an existing instance - during deserialization an existing instance can be provided to reduce GC

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>1.5.0</version>
<scope>provided</scope> </dependency> 

For use in Android, Gradle can be configured with:

apply plugin: 'android-apt' dependencies {

compile 'com.dslplatform:dsl-json:1.5.1'
apt 'com.dslplatform:dsl-json-processor:1.5.0' 
}
 

Project examples can be found in examples folder

Java/DSL property mapping

Java/DSL collection mapping

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

@JsonConverter which implements Configuration will also be registered in META-INF/services which makes it convenient to setup initialization.

@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
• mandatory - mandatory properties must exists in JSON. Even in omit-defaults mode. If property is not found, IOException will be thrown
• hashMatch - DSL-JSON matches properties by hash values. If this option is turned off exact comparison will be performed which will add minor 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
• typeSignature - disable inclusion of $type during abstract type serialization. By default abstract type will include additional information which is required for correct deserialization. Abstract types can be deserialized into a concreted type by defining deserializeAs on @CompiledJson which allows the removal of $type during both serialization and deserialization

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.

Required properties

Jackson required = true can be used to fail if property is missing in JSON:

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
• 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>1.5.1</version> </dependency> 

Best practices

Reusing reader/writer.

JsonWriter It has two modes of operations:

• populating the entire output into byte[]
• targeting output stream and flushing local byte[] to target output stream

JsonWriter can be reused via reset methods which binds it to specified target. When used directly it should be always created via newWriter method on DslJson instance. Several DslJson serialize methods will reuse the writer via thread local variable. When using JsonWriter via the first mode, result can be copied to stream via .toStream(OutputStream) method.

DslJson<Object> json = ... // always reuse OutputStream stream = ... // stream with JSON in UTF-8 json.serialize(pojo, stream);
 //will use thread local writer 

JsonReader can process byte[] or InputStream inputs. It can be reused via the process methods. When calling DslJson deserialize methods often exists in two flavors:

• with byte[] argument, in which case a new JsonReader will be created, but for best performance byte[] should be reused
• without byte[] argument in which case thread local reader will be reused

For small messages it's better to use byte[] API. When reader is used directly it should be always created via newReader method on DslJson instance.

DslJson<Object> json = ... // always reuse InputStream stream = ... // stream with JSON in UTF-8 POJO instance = json.deserialize(POJO.class, stream);
 //will use thread local reader 

Binding

JsonReader has iterateOver method for exposing input collection as consumable iterator. Also, since v1.5 binding API is available which can reuse instances for deserialization.

DslJson<Object> json = new DslJson<Object>();
 //always reuse byte[] bytes = "{
\"number\":123
}
".getBytes("UTF-8");
 JsonReader<Object> reader = json.newReader().process(bytes, bytes.length);
 POJO instance = new POJO();
 //can be reused POJO bound = reader.next(POJO.class, instance);
 //bound is the same as instance above 

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

Q: Annotation processor checks for new DSL compiler version on every compilation. How can I disable that?
A: If you specify custom dsljson.compiler processor option or put dsl-compiler.exe in project root it will use that one and will not check online for updates

Q: What is this DSL Platform?
A: DSL Platform is a proprietary compiler written in C#. It's free to use, but access to source code is licensed. If you need access to compiler or need performance consulting let us know