Dynamically-scoped references offer a more disciplined alternative to global variables. They're a handy feature from the Lisp family of languages.

In Java, static fields are equivalent to global variables, with all the bad stuff that entails. By contrast, you create a DynamicReference with an initial default value, which can be overridden inside a given block scope. Code inside the block (at any call depth) will automatically use the overridden value. After the block finishes, the value reverts back.

Here's an example of overriding a reference to System.out:

import com.statemachinesystems.util.DynamicReference;
import java.io.*;

...

DynamicReference<PrintStream> out = new DynamicReference<>(System.out);

PrintStream log = new PrintStream(new File("out.log"), "UTF-8");

void sayHello() {
    out.get().println("Hello World!");
}

...

// write to standard output
sayHello();

...

out.withValue(log, () -> {
    // write to the log file
    sayHello();
});

This implementation is based on the DynamicVariable class from the Scala standard library.

Getting started

This library is in the Maven Central repo, so just add the following chunk to your pom.xml (or the equivalent for Gradle/SBT/whatever):

<dependency>
    <groupId>com.statemachinesystems</groupId>
    <artifactId>dynamic-reference</artifactId>
    <version>1.0</version>
</dependency>

© 2015 State Machine Systems Ltd. Apache Licence, Version 2.0