根据《单元测试的艺术》一书中的观点，一个良好的单元测试包括三个步骤：

1. 准备测试环境和数据；
2. 执行目标方法；
3. 验证执行结果（判断程序的运行结果是否如你所想）。

我们一般使用断言（Assert）进行结果验证，Junit的org.junit.Assert包提供了大量断言API，如：assertEquals、assertTrue和assertNotNull等等。总体来说，Junit的断言API还可以，功能不算强大，只能说是满足需求。

接下来介绍下本文的主角：AssertJ。AssertJ的slogan是：Fluent assertions for java。如果读者了解构建链模式或者Java 8的stream&filter，那么就可以体会到这种思路的好处了。

接下来看看官网给出的例子（确实比Junit强大很多）：

// unique entry point to get access to all assertThat methods and utility methods (e.g. entry)
import static org.assertj.core.api.Assertions.*;

// in the following examples, fellowshipOfTheRing is a List of [TolkienCharacter](https://github.com/joel-costigliola/assertj-examples/blob/java-8/assertions-examples/src/main/java/org/assertj/examples/data/TolkienCharacter.java)

// basic assertions
assertThat(frodo.getName()).isEqualTo("Frodo");
assertThat(frodo).isNotEqualTo(sauron)
                 .isIn(fellowshipOfTheRing);

// String specific assertions
assertThat(frodo.getName()).startsWith("Fro") 
                           .endsWith("do") 
                           .isEqualToIgnoringCase("frodo");

// collection specific assertions
assertThat(fellowshipOfTheRing).hasSize(9) 
                               .contains(frodo, sam)
                               .doesNotContain(sauron);

// Java 8 exception assertion
assertThatThrownBy(() -> { throw new Exception("boom!"); }).isInstanceOf(Exception.class)
                                                           .hasMessageContaining("boom");

// Java 8 BDD style exception
 assertion Throwable thrown = catchThrowable(() -> { throw new Exception("boom!"); });

assertThat(thrown).isInstanceOf(Exception.class) 
                  .hasMessageContaining("boom");

// using extracting magical feature to check fellowshipOfTheRing characters name :)
assertThat(fellowshipOfTheRing).extracting("name")
                               .contains("Boromir", "Gandalf", "Frodo", "Legolas")
                               .doesNotContain("Sauron", "Elrond");

// Extracting with Java 8 love (type safe)
assertThat(fellowshipOfTheRing).extracting(TolkienCharacter::getName) 
                               .contains("Boromir", "Gandalf", "Frodo", "Legolas")
                               .doesNotContain("Sauron", "Elrond");

// Extracting multiple values at once (using tuple to group them)
assertThat(fellowshipOfTheRing).extracting("name", "age", "race.name")
                               .contains(tuple("Boromir", 37, "Man"),
                                                 tuple("Sam", 38, "Hobbit"),
                                                 tuple("Legolas", 1000, "Elf"));

// filter collection before 
assertionassertThat(fellowshipOfTheRing).filteredOn("race", HOBBIT)
                                        .containsOnly(sam, frodo, pippin, merry);

// filter collection with Java 8 
PredicateassertThat(fellowshipOfTheRing).filteredOn(character -> character.getName().contains("o"))
                                        .containsOnly(aragorn, frodo, legolas, boromir);

// combining filtering and extraction (yes we can)
assertThat(fellowshipOfTheRing).filteredOn(character -> character.getName().contains("o"))
                               .containsOnly(aragorn, frodo, legolas, boromir)
                               .extracting(character -> character.getRace().getName())
                               .contains("Hobbit", "Elf", "Man");

// and many more assertions : map, dates (java 7 and java 8), file, numbers, optional ...

参考资料

1. AssertJ Core latest news
2. JUnit 单元测试断言推荐 AssertJ