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Handling Properties

22-12-2019 - Shai Zambrovski

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Must of Java application need to use with key/value pairs for persist configuration metadata, user information or any data that related to a program or application. So we need to have am intuitive way to manage key/value mechanism to:

• set or get values.
• Set default value for non-exist pairs.

for this, Java provides us the java.util.Properties class that manage the key/value pair as a String.

The Properties class provides us basically two methods:

• setPropertySet value (create or modify) to a specific key.
• getPropertyGet the value of a specific key; null is returned if key doesn’t exists, unless we provide default value.

So how we can persist those properties?

Using streams we can basically persist two types of properties files format; .properties and .xml files.

.properties format

• Can be handled with outputStream & inputStream (if we work with bytes) or reader & writer (if we work with characters).
• Key/value separated with : or =.
• Comments starts with # or !.

Write to .properties file:

Properties props = new Properties();
props.setProperty("courseName", "Java Fundamentals");
props.setProperty("courseLocation", "Auditorium");
try (Writer writer = Files.newBufferedWriter(Paths.get("courses.properties"))) {
   props.store(writer, "2019 courses"); // In case we don't want any comments, need to pass a null in the 2nd argument
}

courses.properties file:

#2019 courses
#Wed Dec 25 14:03:57 IST 2019
courseLocation=Auditorium
courseName=Java Fundamentals

Read from .properties file:

Properties props = new Properties();
try (Reader reader = Files.newBufferedReader(Paths.get("courses.properties"))) {
    props.load(reader);
} catch(...) {}
String name = props.getProperty("courseName"); //Java Fundamentals
String location = props.getProperty("courseLocation"); //Auditorium

.xml format

• Can be handled only with outputStream & inputStream.
• Key will persisted as a key attribute and value will persisted as an element value.
• Comments will persisted as an element.

Write to .xml file:

Properties props = new Properties();
props.setProperty("courseName", "Java Fundamentals");
props.setProperty("courseLocation", "Auditorium");
try (OutputStream os = Files.newOutputStream(Paths.get("courses.xml"))) {
    props.storeToXML(os, "2019 courses"); // In case we don't want any comments, need to pass a null in the 2nd argument
}

courses.xml file:

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE properties SYSTEM "http://java.sun.com/dtd/properties.dtd">
<properties>
    <comment>2019 courses</comment>
    <entry key="courseLocation">Auditorium</entry>
    <entry key="courseName">Java Fundamentals</entry>
</properties>

Read from .xml file:

Properties props = new Properties();
try (InputStream is = Files.newInputStream(Paths.get("courses.xml"))) {
   props.loadFromXML(is);
} catch (...) {}
String name = props.getProperty("courseName"); //Java Fundamentals
String location = props.getProperty("courseLocation"); //Auditorium

Default Properties mechanism

In some cases and scenarios we want to have a default values to provide an abstraction for some configurations.

To achieve such feature, we can pass to the Properties constructor a Properties instance that will supply a default values for a keys that doesn’t exists in the current Properties instance.

Properties defaultProps = new Properties();
defaultProps.setProperty("courseLocation", "classroom A33");
Properties configuration = new Properties(defaultProps);
String location = configuration.getProperty("courseLocation"); // "classroom A33"
configuration.setProperty("courseLocation", "Auditorium"); // Override default value
location = configuration.getProperty("courseLocation"); // "Auditorium"