Requirements API

A fluent API for enforcing design contracts with automatic message generation:

✔️ Easy to use
✔️ Fast
✔️ Production-ready

To get started, add this Maven dependency:

<dependency>
  <groupId>com.github.cowwoc.requirements</groupId>
  <artifactId>java</artifactId>
  <version>10.4</version>
</dependency>

Usage Example

import java.util.List;
import java.util.StringJoiner;

import static com.github.cowwoc.requirements10.java.DefaultJavaValidators.checkIf;
import static com.github.cowwoc.requirements10.java.DefaultJavaValidators.requireThat;
import static com.github.cowwoc.requirements10.java.DefaultJavaValidators.that;

public final class Cake
{
  private byte bitesTaken = 0;
  private int piecesLeft;

  public Cake(int piecesLeft)
  {
    requireThat(piecesLeft, "piecesLeft").isPositive();
    this.piecesLeft = piecesLeft;
  }

  public int eat()
  {
    ++bitesTaken;
    assert that(bitesTaken, "bitesTaken").isNotNegative().elseThrow();

    piecesLeft -= ThreadLocalRandom.current().nextInt(5);

    assert that(piecesLeft, "piecesLeft").isNotNegative().elseThrow();
    return piecesLeft;
  }

  public List<String> getFailures()
  {
    return checkIf(bitesTaken, "bitesTaken").isNotNegative().
      and(checkIf(piecesLeft, "piecesLeft").isGreaterThan(3)).
      elseGetMessages();
  }
}

If you violate a precondition:

Cake cake = new Cake(-1000);

You'll get:

java.lang.IllegalArgumentException: "piecesLeft" must be positive.
actual: -1000

If you violate a class invariant:

Cake cake = new Cake(1_000_000);
while (true)
  cake.eat();

You'll get:

java.lang.AssertionError: "bitesTaken" may not be negative.
actual: -128

If you violate a postcondition:

Cake cake = new Cake(100);
while (true)
  cake.eat();

You'll get:

java.lang.AssertionError: "piecesLeft" may not be negative.
actual: -4

If you violate multiple conditions at once:

Cake cake = new Cake(1);
cake.bitesTaken = -1;
cake.piecesLeft = 2;
StringJoiner failures = new StringJoiner("\n\n");
for (String failure : cake.getFailures())
    failures.add(failure);
System.out.println(failures);

You'll get:

"bitesTaken" may not be negative.
actual: -1

"piecesLeft" must be greater than 3.
actual: 2

Features

This library offers the following features:

• Automatic message generation for validation failures
  • Diffs provided whenever possible to highlight the differences between expected and actual values
  • Clean stack-traces that remove unnecessary frames
  • Zero overhead when assertions are disabled for better performance
  • Multiple validation failures that report all the errors at once
  • Nested validations that allow you to validate complex objects
  • String diff that shows the differences between two strings
  • Performant and robust

Entry Points

Designed for discovery using your favorite IDE's auto-complete feature. The main entry points are:

• requireThat(value, name) for method preconditions.
  • that(value, name) for class invariants, method postconditions and private methods.
  • checkIf(value, name) for multiple failures and customized error handling.

See the API documentation for more details.

Best practices

• Use checkIf().elseGetMessages() to return failure messages without throwing an exception. This is the fastest validation approach, ideal for web services.
  • To enhance the clarity of failure messages, you should provide parameter names, even when they are optional. In other words, favor assert that(value, name) over assert that(value).

Third-party libraries and tools

This library supports the following third-party libraries and tools:

• guava
  • IntelliJ IDEA

Licenses

• This library is licensed under the Apache License, Version 2.0
  • See Third party licenses for the licenses of the dependencies
  • Icons made by Flat Icons from www.flaticon.com are licensed by CC 3.0 BY