properties

Overview

Jackson (Java) data format module that supports reading and writing Java Properties files, using naming convention to determine implied structure (by default assuming dotted notation, but configurable from non-nested to other separators).

Status

Since version 2.9 this module is considered complete and production ready. All Jackson layers (streaming, databind, tree model) are supported.

Maven dependency

To use this extension on Maven-based projects, use following dependency:

<dependency>
  <groupId>com.fasterxml.jackson.dataformat</groupId>
  <artifactId>jackson-dataformat-properties</artifactId>
  <version>2.13.4</version>
</dependency>

Usage

Basic usage is by using JavaPropsFactory in places where you would usually use JsonFactory; or, for convenience, JavaPropsMapper instead of plain ObjectMapper.

JavaPropsMapper mapper = new JavaPropsMapper();
// and then read/write data as usual
SomeType value = ...;
String props = mapper.writeValueAsBytes(value);
// or
mapper.writeValue(new File("stuff.properties", value);
SomeType otherValue = mapper.readValue(props, SomeType.class);

For reading and writing, you most commonly use ObjectReader and ObjectWriter, created using mapper object you have; for example:

String props = mapper.writerFor(Pojo.class)
    .writeValueAsString(pojo);
Pojo result = mapper.readerFor(Pojo.class)
    .with(schema) // if customization needed
    .readValue(propsFile);

Basics of conversion

Since default java.util.Properties can read "flat" key/value entries in, what is the big deal here?

Most properties files actually use an implied structure by using a naming convention; most commonly by using period ('.') as logical path separator. So you may have something like:

title=Home Page
site.host=localhost
site.port=8080

to group related properties together. This implied structure could easily be made actual explicit Structure: for example, we could consider following Class definitions

public class WebPage {
  public String title;
  public Site site;
}
static class Site {
  public String host;
  public int port;
}

So what this module does is to convert "flattened" properties keys into hierarchic structure and back; and by doing this allows Jackson databinding to bind such properties content into full hierarcic Object model.

Simple POJO serialization

Let's consider another simple POJO definition of:

public class User {
    public Gender gender;
    public Name name; // nested object
    public boolean verified;
    public byte[] userImage;
}
static class Name {
    public String first, last;
}
enum Gender { M, F, O; }

and code

User user = new User(Gender.M, new Name("Bob", "Palmer"),
   true, new byte[] { 1, 2, 3, 4 });
String propStr = mapper.writeValueAsString(user);

and with that, we could have following contents in String propStr:

gender=M
name.first=Bob
name.last=Palmer
verified=true
userImage=AQIDBA==

Simple POJO deserialization

Given a String of propStr, we could easily read contents back as a POJO with:

User result = mapper.readValue(props, User.class);

and verify that contents are as expected.

Basic array handling

Although path notation usually assumes that path segments (pieces between separators, comma by default) are to be considered logical POJO properties (no pun intended), or fields, there is default handling (which may be disabled) to infer index values from path segments that are simple numbers. This means that default handling for Properties content like:

boxes.1.x = 5
boxes.1.y = 6
boxes.2.x = -5
boxes.2.y = 15

could be easily read into following POJO:

public class Container {
  public List<Box> boxes;
}
static class Box {
  public int x, y;
}

with

Container c = mapper.readValue(propsFile, Boxes.class);

Similarly when writing out Properties content, default inclusion mechanism is to use "simple" indexes, and to start with index 1. Note that most of these aspects are configurable; and note also that when reading, absolute value of index is not used as-is but only to indicate ordering of entries: that is, gaps in logical numbering do not indicate null values in resulting arrays and Lists.

Using "Any" properties

TO BE WRITTEN

Customizing handling with JavaPropsSchema

The most important format-specific configuration mechanism is the ability to define a FormatSchema (type define in jackson-core) to use for parsing and generating content in specific format. In case of Java Properties, schema type to use is JavaPropsSchema.

Format definitions may be passed either directly to specific JavaPropsGenerator / JavaPropsParser (when working directly with low-level Streaming API), or, more commonly, when constructing ObjectReader / ObjectWriter (as shown in samples above). If no schema is explicitly specified, the default instance (accessible with JavaPropsSchema.emptySchema()) is used.

Schema instances are created by using "mutant factory" methods like withPathSeparator on an instance; since schema instances are fully immutable (to allow thread-safe sharing and reuse), a new instance is always created if settings would need to change. Typical usage, then is:

JavaPropsSchema schema = JavaPropsSchema.emptySchema()
   .withPathSeparator("->");
Pojo stuff = mapper.readerFor(Pojo.class)
   .with(schema)
   .readValue(source);
// and writing
mapper.writer(schema)
   .writeValue(stuff, new File("stuff.properties");

Currently existing configuration settings to use can be divide into three groups:

• Separators for indicating how components of logical entries are separated from each other. Some settings apply to both reading and writing, others to only generation
• Array settings for indicating how flat Property keys can be interpreted as hierarchic paths
• Other settings for things that are neither separators nor array settings.

JavaPropsSchema: separators

JavaPropsSchema.keyValueSeparator

• Marker used to separate property key and value segments when writing content
  • Only affects writing: currently (2.7) has no effect on reading
• Default value: "="
• Mutator method: JavaPropsSchema.withKeyValueSeparator(String)

JavaPropsSchema.lineEnding

• String output after each logical key/value entry, usually linefeed
  • Only affects writing: currently (2.7) has no effect on reading (reader assumes a linefeed)
• Default value: "\n" (Unix linefeed)
• Mutator method: JavaPropsSchema.withLineEnding(String)

JavaPropsSchema.lineIndentation

• String output before each logical key/value entry, useful for adding indentation
  • Only affects writing: currently (2.7) has no effect on reading (reader skips any white space that precedes key name)
• Default value: "" (empty String; that is, no indentation prepended)
• Mutator method: JavaPropsSchema.withLineIndentation(String)

JavaPropsSchema.pathSeparator

• Marker used to separate logical path segments within key name, if any; if disabled (specified as empty String), no path component separation is performed
  • Affects both reading and writing
• Default value: "."
• Mutator methods
  • JavaPropsSchema.withPathSeparator(String) to assign path separator (except if "" given, same as disabling)
  • JavaPropsSchema.withoutPathSeparator() to disable use of path logic; if so, only main-level properties are available, with exact property key as name

JavaPropsSchema.pathSeparatorEscapeChar

• Marker used to enable the JavaPropsSchema.pathSeparator to be included in key names.
• Default value: '\0' (effectively disabling it).
• Mutator methods
  • JavaPropsSchema.withPathSeparatorEscapeChar(char) to assign path separator escape char
• Notes
  • The escape character is only used if the path separator is a single character.
  • The escape character is only used for escaping either the pathSeparator character or a sequence of escape characters immediately prior to the pathSeparator.
  • Any escape character may be used.
  • Backslash ('\') is the most obvious character to use, but be aware that the JDK Properties loader has its own rules for escape processing (documented in the Javadoc for [Properties.load] (https://docs.oracle.com/en/java/javase/18/docs/api/java.base/java/util/Properties.html#load(java.io.Reader) ) ) that will remove ALL duplicated backslash characters (and also carry out other escape handling) before the JavaPropsMapper gets to see them.
• Examples
  • Given a pathSeparator of "." and an escape char of '#' then
    • a#.b produces a segment called "a.b"
    • a##.b produces a segment called "a#" with a child called "b"
    • a###.b produces a segment called "a#.b"
    • a#b produces a segment called "a#b" - the escape processing is only used immediately prior to the path separator.

JavaPropsSchema: array representation

JavaPropsSchema.firstArrayOffset

• If array handling is enabled (simple and/or marker-based indexes), defines the value used as path component for the first element; other elements advance index value by 1
  • Has only effect on writing: when reading, index values are used for ordering but are not used as absolute physical indexes within logical arrays
  • Most commonly used values are 1 and 0, although any integer value may be used
• Default value: 1
• Mutator method: JavaPropsSchema.withFirstArrayOffset(int)

JavaPropsSchema.indexMarker

• Optional pair of start- and end-markers that may be used to denote array entries
• Default value: Markers("[", "]") -- allows use of notation like "path.array[1].x = 15"
  • Means that by default marker-based notation IS allowed
• Mutator methods:
  • JavaPropsSchema.withIndexMarker(Markers)
  • JavaPropsSchema.withoutIndexMarker()

JavaPropsSchema.parseSimpleIndexes

• On/off setting that determines whether path segments that are valid positive integers are automatically considered to indicate an array value
  • As name implies, only affects reading
• Default value: true -- means that by default path segments that "look like array index" will be considered to be array entries
• Mutator method: JavaPropsSchema.withParseSimpleIndexes(boolean)

JavaPropsSchema.writeIndexUsingMarkers

• On/off setting that determines which notation (simple, or marker-based) is used for writing paths for array values: if true, marker-based (start/end markers) indexing is used, if false, "simple" notation where index is output as path segment
  • With default settings these would lead to either "path.array.1.x = 15" (false) or "path.array[1].x = 15" output
• Default value: false (so "simple" notation is used, "path.array.1.x = 15")
• Mutator method: JavaPropsSchema.withWriteIndexUsingMarkers(boolean)

JavaPropsSchema: other configuration

JavaPropsSchema.header

• Optional String to output at the beginning of resulting Properties output, before any of the values is output; typically to add comment line(s)
  • NOTE: should always end with a linefeed (unless specified as empty String), as otherwise first entry would be concatenated as part of last line of header
• Default value: "" (empty String, that is, no output)
• Mutator method: JavaPropsSchema.withHeader(String)

Sample Usage

ZooKeeper Configs

Consider following simple ZooKeeper config file from ZK Documentation:

tickTime=2000
dataDir=/var/zookeeper
clientPort=2181
initLimit=5
syncLimit=2
server.1=zoo1:2888:3888
server.2=zoo2:2888:3888
server.3=zoo3:2888:3888

this could be expressed by, say, following Java classes (used by unit tests of this module):

static class ZKConfig {
    public int tickTime;
    public File dataDir;
    public int clientPort;
    public int initLimit, syncLimit;
    @JsonProperty("server")
    public List<ZKServer> servers;
}

static class ZKServer {
  public final int srcPort, dstPort;
  public final String host;

  @JsonCreator
  public ZKServer(String combo) { // should validate better; should work for now
    String[] parts = combo.split(":");
	host = parts[0];h
    srcPort = Integer.parseInt(parts[1]);
    dstPort = Integer.parseInt(parts[2]);
  }

  @JsonValue
  public String asString() {
    return String.format("%s:%d:%d", host, srcPort, dstPort);
  }
}

where the only slightly complicated part is splitting of colon-separated endpoint definition. As earlier, reading such Configuration would be as simple as:

ZKConfig config = propsMapper.readValue(new File("zook.properties"), ZKConfig.class);

after which access to properties would be done using simple field access (or, if we prefer, additional getters).

Note: in this example, default schema configuration worked so we did not have to set it for reading. If we did, we would have used something like:

JavaPropsSchema schema = JavaPropsSchema.emptySchema()
   .withPathSeparator("->");
propsMapper.writer(schema)
   .writeValue(config, new File("zook-modified.properties");
// and similarly when reading