Add AssertJ checkstyle rule

Closes gh-5083

spring-boot-parent/src/checkstyle/checkstyle.xml

@@ -71,7 +71,7 @@
 		<module name="AvoidStarImport" />
 		<module name="AvoidStaticImport">
 			<property name="excludes"
-				value="org.junit.Assert.*, org.junit.Assume.*, org.junit.internal.matchers.ThrowableMessageMatcher.*, org.hamcrest.CoreMatchers.*, org.hamcrest.Matchers.*, org.springframework.boot.configurationprocessor.ConfigurationMetadataMatchers.*, org.springframework.boot.configurationprocessor.TestCompiler.*, org.mockito.Mockito.*, org.mockito.BDDMockito.*, org.mockito.Matchers.*, org.springframework.test.web.servlet.request.MockMvcRequestBuilders.*, org.springframework.test.web.servlet.result.MockMvcResultMatchers.*, org.springframework.security.test.web.servlet.request.SecurityMockMvcRequestPostProcessors.*, org.springframework.security.test.web.servlet.setup.SecurityMockMvcConfigurers.*, org.springframework.hateoas.mvc.ControllerLinkBuilder.linkTo" />
+				value="org.assertj.core.api.Assertions.*, org.junit.Assert.*, org.junit.Assume.*, org.junit.internal.matchers.ThrowableMessageMatcher.*, org.hamcrest.CoreMatchers.*, org.hamcrest.Matchers.*, org.springframework.boot.configurationprocessor.ConfigurationMetadataMatchers.*, org.springframework.boot.configurationprocessor.TestCompiler.*, org.mockito.Mockito.*, org.mockito.BDDMockito.*, org.mockito.Matchers.*, org.springframework.test.web.servlet.request.MockMvcRequestBuilders.*, org.springframework.test.web.servlet.result.MockMvcResultMatchers.*, org.springframework.security.test.web.servlet.request.SecurityMockMvcRequestPostProcessors.*, org.springframework.security.test.web.servlet.setup.SecurityMockMvcConfigurers.*, org.springframework.hateoas.mvc.ControllerLinkBuilder.linkTo" />
 		</module>
 		<module name="IllegalImport" />
 		<module name="RedundantImport" />
@@ -136,6 +136,13 @@
 				value="Please use BDDMockto imports." />
 			<property name="ignoreComments" value="true" />
 		</module>
+ 		<module name="RegexpSinglelineJava">
+ 			<property name="maximum" value="0"/>
+			<property name="format" value="org\.junit\.Assert\.assert" />
+			<property name="message"
+				value="Please use AssertJ imports." />
+			<property name="ignoreComments" value="true" />
+		</module>
 		<module name="Regexp">
 			<property name="format" value="[ \t]+$" />
 			<property name="illegalPattern" value="true" />

spring-boot/src/main/java/org/springframework/boot/test/OutputCapture.java

@@ -24,6 +24,7 @@ import java.util.ArrayList;
 import java.util.List;
 
 import org.hamcrest.Matcher;
+import org.junit.Assert;
 import org.junit.rules.TestRule;
 import org.junit.runner.Description;
 import org.junit.runners.model.Statement;
@@ -32,7 +33,6 @@ import org.springframework.boot.ansi.AnsiOutput;
 import org.springframework.boot.ansi.AnsiOutput.Enabled;
 
 import static org.hamcrest.Matchers.allOf;
-import static org.junit.Assert.assertThat;
 
 /**
  * JUnit {@code @Rule} to capture output from System.out and System.err.
@@ -63,7 +63,7 @@ public class OutputCapture implements TestRule {
 					try {
 						if (!OutputCapture.this.matchers.isEmpty()) {
 							String output = OutputCapture.this.toString();
-							assertThat(output, allOf(OutputCapture.this.matchers));
+							Assert.assertThat(output, allOf(OutputCapture.this.matchers));
 						}
 					}
 					finally {