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之前一直使用Junit自带的Assert类进行断言，尽管这能满足一些我们最基础的需要，但从功能上来讲还是不够强大的。

今天介绍一款功能强大的流式断言器AssertJ，所谓的流式断言就是相较于Assert的单个校验点断言，支持一条断言语句对实际值同时断言多个校验点。

官方地址：http://joel-costigliola.github.io/assertj/
开源地址：https://github.com/joel-costigliola/assertj-core

从官网上我们可以看到AssertJ支持的库有这些，具体用法上官方查询：

Core：AssertJ core is a Java library that provides a fluent interface for writing assertions.

Assertions generator：Use the Assertion Generator to create assertions specific to your own classes.

Guava：AssertJ assertions for Guava provides assertions for Guava types like Multimap, Table, Optional, Range or ByteSource.

Joda-Time：AssertJ assertions for Joda-Time provides assertions for Joda-Time types like DateTime and LocalDateTime.

DB：AssertJ-DB provides assertions to test data in a database.

Neo4j：Provides assertions for Neo4j 2 or higher.

Swing：AssertJ Swing is a Java library that provides a fluent interface for functional Swing UI testing.

maven引入依赖如下：

<dependency><groupId>org.assertj</groupId><artifactId>assertj-core</artifactId><!-- use 3.0.0 for Java 8 projects --><!-- 2.0.0 required Java 7 --><!-- 1.7.1 for Java 6 --><version>1.7.1</version><scope>test</scope>
</dependency>

此外对应的开发库引入如下：

<dependency><groupId>org.assertj</groupId><artifactId>assertj-joda-time</artifactId><version>1.1.0</version><scope>test</scope></dependency><dependency><groupId>org.assertj</groupId><artifactId>assertj-guava</artifactId><version>2.0.0</version><scope>test</scope><exclusions><exclusion><groupId>org.assertj</groupId><artifactId>assertj-core</artifactId></exclusion></exclusions></dependency><dependency><groupId>org.assertj</groupId><artifactId>assertj-neo4j</artifactId><version>1.0.0</version><scope>test</scope></dependency><dependency><groupId>org.assertj</groupId><artifactId>assertj-db</artifactId><version>1.0.1</version><scope>test</scope></dependency>

官方上的用法介绍：

// 为所有assertThat和工具方法提供唯一访问入口（例如entry）
import static org.assertj.core.api.Assertions.*;// 下面的示例中，fellowshipOfTheRing是一个TolkienCharacter列表// 简单断言
assertThat(frodo.getName()).isEqualTo("Frodo");
assertThat(frodo).isNotEqualTo(sauron).isIn(fellowshipOfTheRing);// String断言
assertThat(frodo.getName()).startsWith("Fro").endsWith("do").isEqualToIgnoringCase("frodo");// 集合断言
assertThat(fellowshipOfTheRing).hasSize(9).contains(frodo, sam).doesNotContain(sauron);// 使用extracting特性检查fellowshipOfTheRing中的名字:)
assertThat(fellowshipOfTheRing).extracting("name").contains("Boromir", "Gandalf", "Frodo", "Legolas").doesNotContain("Sauron", "Elrond");// Java 8方式的extracting
assertThat(fellowshipOfTheRing).extracting(character -> character.getRace().getName()).contains("Hobbit", "Elf")               .doesNotContain("Orc");                                                 // 断言之前过滤集合
assertThat(fellowshipOfTheRing).filteredOn("race", HOBBIT).containsOnly(sam, frodo, pippin, merry);// 使用Java 8 lambda predicate过滤集合
assertThat(fellowshipOfTheRing).filteredOn(character -> character.getName().contains("o")).containsOnly(aragorn, frodo, legolas, boromir);// 将extraction和过滤结合（是的，我们可以做到）
assertThat(fellowshipOfTheRing).filteredOn(character -> character.getName().contains("o")).containsOnly(aragorn, frodo, legolas, boromir).extracting(character -> character.getRace().getName()).contains("Hobbit", "Elf", "Man");

个人常用到的一些方法：
String 校验

    @Testpublic void testString() {String str = null;// 断言null或为空字符串assertThat(str).isNullOrEmpty();// 断言空字符串assertThat("").isEmpty();// 断言字符串相等 断言忽略大小写判断字符串相等assertThat("Frodo").isEqualTo("Frodo").isEqualToIgnoringCase("frodo");// 断言开始字符串 结束字符穿 字符串长度assertThat("Frodo").startsWith("Fro").endsWith("do").hasSize(5);// 断言包含字符串 不包含字符串assertThat("Frodo").contains("rod").doesNotContain("fro");// 断言字符串只出现过一次assertThat("Frodo").containsOnlyOnce("do");// 判断正则匹配assertThat("Frodo").matches("..o.o").doesNotMatch(".*d");}

数字校验

    @Testpublic void testNumber() {Integer num = null;// 断言空assertThat(num).isNull();// 断言相等assertThat(42).isEqualTo(42);// 断言大于 大于等于assertThat(42).isGreaterThan(38).isGreaterThanOrEqualTo(38);// 断言小于 小于等于assertThat(42).isLessThan(58).isLessThanOrEqualTo(58);// 断言0assertThat(0).isZero();// 断言正数 非负数assertThat(1).isPositive().isNotNegative();// 断言负数 非正数assertThat(-1).isNegative().isNotPositive();}

时间断言

@Test  public void testDate() {  // 断言与指定日期相同 不相同 在指定日期之后 在指定日期之钱  assertThat(Dates.parse("2014-02-01")).isEqualTo("2014-02-01").isNotEqualTo("2014-01-01")  .isAfter("2014-01-01").isBefore(parse("2014-03-01"));  // 断言 2014 在指定年份之前 在指定年份之后  assertThat(DateTime.now().toDate()).isBeforeYear(2020).isAfterYear(2013);  // 断言时间再指定范围内 不在指定范围内  assertThat(parse("2014-02-01")).isBetween("2014-01-01", "2014-03-01").isNotBetween(  parse("2014-02-02"), parse("2014-02-28"));  

List断言

    @Test public void testList() {// 断言 列表是空的assertThat(newArrayList()).isEmpty();// 断言 列表的开始 结束元素assertThat(newArrayList(1, 2, 3)).startsWith(1).endsWith(3);// 断言 列表包含元素 并且是排序的assertThat(newArrayList(1, 2, 3)).contains(1, atIndex(0)).contains(2, atIndex(1)).contains(3).isSorted();// 断言 被包含与给定列表assertThat(newArrayList(3, 1, 2)).isSubsetOf(newArrayList(1, 2, 3, 4));// 断言 存在唯一元素assertThat(Lists.newArrayList("a", "b", "c")).containsOnlyOnce("a");}

Map断言

    @Testpublic void testMap() {Map<String, Object> foo = Maps.newHashMap();foo.put("A", 1);foo.put("B", 2);foo.put("C", 3);// 断言 map 不为空 sizeassertThat(foo).isNotEmpty().hasSize(3);// 断言 map 包含元素assertThat(foo).contains(entry("A", 1), entry("B", 2));// 断言 map 包含keyassertThat(foo).containsKeys("A", "B", "C");// 断言 map 包含valueassertThat(foo).containsValue(3);}

添加错误提示信息： as

assertThat("abc").as("校验abc").isEqualTo("abcd");错误提示：
org.junit.ComparisonFailure: [校验abc] expected:<"abc[d]"> but was:<"abc[]">

链式校验

// isEqualTo 相等， contains包含
assertThat("abc").as("校验abc").isEqualTo("abc").contains("d");错误提示：
FAILED: testAssertJ
java.lang.AssertionError: [校验abc] 
Expecting:<"abc">
to contain:<"d"> 

大小校验

List list = new ArrayList();assertThat(list).hasSize(1);
FAILED: testAssertJ
java.lang.AssertionError: 
Expected size:<1> but was:<0> in:
<[]>

null判断

Object object = null;
assertThat(object).isNotNull();
assertThat(object).isNull();

同时，AssertJ还支持自定义断言，就是根据自己的需求，自定义断言类型。