Source code

This page describes some coding conventions applied in Apache SIS development.

License header

All Java source files (*.java) shall begin with the current ASF license header as described in ASF Source Header. Properties source files (*.properties) used as inputs to some processor (e.g. the resource compiler) shall have the same license header, but with lines prefixed by # instead of *. Properties files distributed as-is in the JAR files can summarize the license on a single line for saving space, as below:

# Licensed to the Apache Software Foundation (ASF) under one or more contributor license agreements.

Naming convention

Classes that do not implement an interface are usually not prefixed, even if abstract. Classes implementing GeoAPI interfaces usually (but not always) begin with Abstract, Default, Simple or General prefix.

  • The Abstract prefix is used when a class is abstract according ISO specifications — it may or may not be be abstract in the Java sense.
  • The General prefix is used when an implementation is designed for use in the general case, as opposed to other implementations specialized for a fixed number of dimensions or other conditions.
  • Implementations specialized for a fixed number of dimensions are suffixed with 1D, 2D, 3D or 4D rather than being prefixed.

Example: the GeneralEnvelope class is an implementation of Envelope interface for the multi-dimensional case. The Envelope2D class is another implementation of the same interface specialized for the two-dimensional case.

Internal packages

All classes in org.apache.sis.internal sub-packages are for SIS usage only and may change without warning in any future release. Those classes are excluded from Javadoc and will not be exported by SIS Jigsaw modules. Those packages may be renamed after SIS upgraded to JDK 9+.

Substitution for non-existent classes

When using a JDK 9+ class that does not exist on JDK 8, define a class of the same name in a org.apache.sis.internal sub-package with the minimal amount of needed functionalities, provided that it can be done with reasonable effort. Otherwise just delete the JDK9-dependent code from the development branch.

Code formatting

Apache SIS uses the standard Java conventions, except for the 80 characters line length restriction. The conventions listed below are guidelines. Some exceptions to those conventions can occur but should be rare (see exceptions to coding conventions).

For making merges between branches easier, refrain from doing massive code reformatting unless:

  • the modified files do not yet exist on the other branches;
  • or the modified lines are known to be identical on all active branches (merges work well in such cases);
  • or the committer is willing to resolve the merge conflicts.

Import statements

Isolate at the end of the imports section any import statements that are specific to a platform. This separation allows any branch to re-arrange the common import statements without generating conflicts with the platform-dependent import statements. Example:

import java.util.List;
import org.opengis.metadata.Metadata;

// Branch-specific imports
import org.opengis.feature.Feature;

Spaces and line length

  • Indentation: Use a consistent space indents and never use tabs.
    • Use 4 space indents for Java files.
    • Use 2 space indents for XML files, because ISO/OGC XML schemas tend to have a very deep structure.
  • Space after keyword: Put a space after if, else, for, try, catch and similar keywords (not after method names).
  • Trailing Whitespaces: Remove all trailing whitespaces.
    • Eclipse users can use the Source - Cleanup option to accomplish this.
    • NetBeans users can use the use the Source - Remove trailing spaces on a file-by-file basis, or set the Preferences - Editor - On Save - Remove trailing whitespaces option.
  • Line wrapping: Use 120-column line width for Java code and Javadoc. Some exceptions to this rule may exist for preserving tabular structures, but should be rare.


  • Curly brackets: The { and } brackets are mandatory for if, else, while and other blocks, except if the instruction after the keyword is on the same line (e.g. else if).

Member declarations

  • Class, method and field declarations shall use the keywords in the following order. This is known as the “customary order” in the Java Language Specification:
    • public, protected or private,
    • abstract or static,
    • final,
    • strictfp (should be applied on all test classes).
  • Member fields do not have any particular prefix (no m_ prefix).

Exceptions to coding conventions

Many mathematical operations in a two-dimensional space (or more) have symmetry. Formatting the code in a way that emphase their symmetrical nature, for example aligning identical terms in columns, can help to understand the overall pattern and to identify bugs. Example:

if (x < xmin) xmin = x;
if (x > xmax) xmax = x;
if (y < ymin) ymin = y;
if (y > ymax) ymax = y;

The decision to use standard or tabular format is made on a case-by-case basis. Of course, tabular format shall not be abused.

Documentation formatting

Apache SIS uses the standard Javadoc conventions, except for the 80 characters line length restriction. Javadoc lines should not exceed 120 characters, but exceptions to this rule may exist for preserving the structure of <table> elements.

Javadoc annotations

SIS uses standard javadoc annotations. The meaning of some tags are refined as below:

  • @since - the SIS version when the annotated element (class, method, etc.) was first introduced.
  • @version - the last SIS version when the code of the annotated class got a significant change.
  • @author - developer name in FirstName LastName (Organization) format. A separated @author tag is added for each developer. The intent is to allow other developers to know to who to ask questions if needed.

In addition, the sis-build-helper modules provides the following custom javadoc taglets:

Javadoc tag Description
@module Create links to the module where the class is defined, source code and Maven artifacts.
{@include} Include the content of a given HTML file below a <h2> section having the given title.
{@preformat} An inline taglet for pre-formatted text. The first word inside the taglet shall be one of java, xml, sql, wkt, text, math or shell.

HTML elements

HTML tags and entities shall be used only when there is no equivalent Javadoc tag. For example:

  • Instead of “<code>✎</code>”, use “{@code ✎}”.
  • Instead of “a &lt; b &lt; c”, use “{@literal a < b < c}”.
  • Instead of “<pre>✎</pre>” for a Java listing, use “{@preformat java ✎}” (this Javadoc tag is specific to Apache SIS — see above table).


Usages of the <p> tag should be relatively rare, since we use CSS styles (see below) as much as possible for controlling the margin between elements like lists and tables. Do not use <p> for the first paragraph in a package, class or member documentation, or for the first paragraph after a </ul>, </ol>, </table>, </blockquote>, </pre>, or {@preformat} element. The <p> tag shall be used only for separating a new paragraph from a previous one. In such cases, <p> shall have a matching </p> tag at the paragraph end in order to form valid HTML.

Javadoc CSS

Avoid using HTML attributes other than class as much as possible. Instead, rely on styling. Some HTML tags having a style definition in Apache SIS are:

HTML tag Description
<div class="note"> Indented text with smaller font, used for notes or examples.
<div class="warning"> Text in red color, used for warning about probable API changes.
<ul> and <ol> Default list styles with few space between items (suitable for compact lists).
<ul class="verbose"> A list style with space between items. Used for lists having verbose (multi-lines) items.
<table class="compact"> Table without border and no space between rows. Used for lists with columns alignment.
<table class="sis"> Table with a border, blue headers, light background and some space between columns.
<th class="hsep"> In SIS tables, draw a line on the top border. Used for drawing table section separators.
<th class="sep"> In SIS tables, draw a bright line on the left border. Used for drawing column separators.
<td class="sep"> In SIS tables, draw a bright line on the left border. Used for drawing column separators.

MathML elements

The source code occasionally uses MathML for formulas that are difficult to render with only Unicode characters. PNG images are not extensively used for formulas because they are difficult to edit after creation, and their content are invisible to search operations (for example when a variable is renamed). For examples of MathML usage in SIS, search for the <math …> XML tag in Java source files (note: there is also legacy {@preformat math …} custom Javadoc tags, but they may be phased out as MathML adoption increase). For an introduction to MathML, see:

MathML is supported natively in Firefox, Safari and Opera. Internet Explorer users need to install a plugin. Firefox users can optionally install the fonts for Mozilla’s MathML engine for better results. Note that a JavaScript display engine is available for all browsers, but not yet used by SIS.