Merge pull request #11320 from nklmish:gh-11241

* pr/11320:
  Polish "Handle null Principal in AuditEvent"
  Handle null Principal in AuditEvent

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/audit/AuditEvent.java

@@ -85,10 +85,9 @@ public class AuditEvent implements Serializable {
 	public AuditEvent(Date timestamp, String principal, String type,
 			Map<String, Object> data) {
 		Assert.notNull(timestamp, "Timestamp must not be null");
-		Assert.notNull(principal, "Principal must not be null");
 		Assert.notNull(type, "Type must not be null");
 		this.timestamp = timestamp;
-		this.principal = principal;
+		this.principal = principal != null ? principal : "";
 		this.type = type;
 		this.data = Collections.unmodifiableMap(data);
 	}
@@ -117,7 +116,8 @@ public class AuditEvent implements Serializable {
 	}
 
 	/**
-	 * Returns the user principal responsible for the event.
+	 * Returns the user principal responsible for the event or an empty String if the
+	 * principal is not available.
 	 * @return the principal
 	 */
 	public String getPrincipal() {

spring-boot-actuator/src/test/java/org/springframework/boot/actuate/audit/AuditEventTests.java

@@ -56,18 +56,18 @@ public class AuditEventTests {
 	}
 
 	@Test
-	public void nullTimestamp() throws Exception {
-		this.thrown.expect(IllegalArgumentException.class);
-		this.thrown.expectMessage("Timestamp must not be null");
-		new AuditEvent(null, "phil", "UNKNOWN",
+	public void nullPrincipalIsMappedToEmptyString() {
+		AuditEvent auditEvent = new AuditEvent(null, "UNKNOWN",
 				Collections.singletonMap("a", (Object) "b"));
+		assertThat(auditEvent.getPrincipal()).isEmpty();
 	}
 
 	@Test
-	public void nullPrincipal() throws Exception {
+	public void nullTimestamp() throws Exception {
 		this.thrown.expect(IllegalArgumentException.class);
-		this.thrown.expectMessage("Principal must not be null");
-		new AuditEvent(null, "UNKNOWN", Collections.singletonMap("a", (Object) "b"));
+		this.thrown.expectMessage("Timestamp must not be null");
+		new AuditEvent(null, "phil", "UNKNOWN",
+				Collections.singletonMap("a", (Object) "b"));
 	}
 
 	@Test