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Aalto XML processor is an ultra-high performance next generation Stax XML processor implementation, implementing both basic Stax API ( and Stax2 API extension (org.codehaus.woodstox.stax2). In addition, it also implements SAX2 API.

In additional to standard Java XML interfaces, one unique feature not implemented by any other Java XML parser that we are aware is so-called non-blocking (asynchronous) XML parsing: ability to parse XML without using blocking I/O, necessary for fully asynchronous processing such as those with Akka framework. Aalto non-blocking API is a minimalistic extension above Stax/Stax2 API to allow indication of "not yet available" token (EVENT_INCOMPLETE) as well as feeding of input (since InputStream can not be used as it blocks)

Aalto is licensed under Apache License 2.0


Build Status Javadoc Tidelift


There are 2 types of support available:


For simple bug reports and fixes, and feature requests, please simply use projects Issue Tracker, with exception of security-related issues for which we recommend filing a Tidelift security contact (NOTE: you do NOT have to be a subscriber to do this).


JDK Compatibility

Aalto 1.x:

  • Can be used on JDK versions 6 (1.6) and up
    • needs JDK 8 or higher to build as of Aalto 1.2.0
  • Contains Java 9 Module definition (module-info.class) starting with version 1.2.0



Aalto dependency is usually added via Maven repository, so something like:


Blocking XML parsing (Stax, SAX)

Blocking XML parsing is done using one of standard interfaces:

  • Stax ( interface -- countless tutorials exist.
    • Also implements Stax2 extension
    • StaxMate is a good companion library for more convenient access

Non-blocking ("async") XML parsing

Non-blocking parsing interface is extension of basic Stax (and Stax2) API, with extensions defined in 'com.fasterxml.aalto' package:

  • AsyncXMLInputFactory offers factory methods for creating non-blocking parsers
  • AsyncXMLStreamReader is extended type that non-blocking parsers implement
    • AsyncXMLStreamReader.EVENT_INCOMPLETE (value 257; just outside range reserved by Stax API) is used to denote "not yet available" (without more data)
    • Method getInputFeeder() is used to access object of type 'AsyncInputFeeder' used to feed input non-blocking way
  • AsyncInputFeeder contains methods for feeding input.

Typical usage pattern is one where block of input is fed to parser, and zero or more complete events are read using basic '' method; and once 'EVENT_INCOMPLETE' is returned, more input needs to be given. AsyncXMLStreamReader itself does not buffer input beyond a single block; caller is responsible for additional buffering, if any. See Async parsing for details.

Construction of AsyncXMLInputFactory is simple; instance may be constructed with or without initial content to parse:

AsyncXMLInputFactory f = new InputFactoryImpl();
// IF there is content use this:
AsyncXMLStreamReader<AsyncByteArrayFeeder> parser = f.createAsyncFor(byteArray);
// If NO CONTENT yet available, just use this:
AsyncXMLStreamReader<AsyncByteArrayFeeder> parser = f.createAsyncForByteArray();

and more content is feed via AsyncInputFeeder when getting EVENT_INCOMPLETE via

parser.getInputFeeder().feedInput(b, offset, dataLength);

or, if no more input available, indicate end-of-content with


See the Full non-blocking parsing example on Wiki for more details.

Aalto Design goals

  • Ultra-high performance parsing by making the Common Case Fast (similar to original RISC manifesto). This may mean limiting functionality, but never compromising correctness. XML 1.0 compliancy is not sacrificed for speed.
  • Allow non-block, asynchronous parsing: it should be possible to "feed" more input and incrementally get more XML events out, without forcing the current thread to block on I/O read operation.



Ultra-high performance non-blocking XML processor (Stax API + extensions)



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