Introduce Error Prone

build.gradle

@@ -33,8 +33,12 @@ buildscript {
         maven { url = sonatypeSnapshots }
 
         jcenter()
+
         maven { url = googleMavenCentralMirror }
 
+        // Repository for error-prone plugin.
+        maven { url = gradlePlugins }
+
         mavenCentral()
         mavenLocal()
 
@@ -50,6 +54,9 @@ buildscript {
                   name: 'guava',
                   version: guavaVersion,
                   changing: true
+        classpath ("net.ltgt.gradle:gradle-errorprone-plugin:$errorPronePluginVersion") {
+            exclude group: 'com.google.guava'
+        }
         classpath("com.google.protobuf:protobuf-gradle-plugin:$protobufGradlePluginVersion") {
             // exclude an old Guava version
             exclude group: 'com.google.guava'
@@ -65,6 +72,14 @@ buildscript {
             exclude group: 'com.google.guava'
         }
     }
+
+    configurations.all({
+        resolutionStrategy {
+            cacheChangingModulesFor(0, 'seconds')
+            force "com.google.guava:guava:$guavaVersion"
+            force "com.google.guava:guava-testlib:$guavaVersion"
+        }
+    })
 }
 
 apply from: 'ext.gradle'
@@ -143,6 +158,7 @@ subprojects {
 
     apply plugin: 'com.google.protobuf'
     apply plugin: 'java'
+    apply plugin: 'net.ltgt.errorprone'
 
     // Specific setup for a Travis build,
     // which prevents appearing of warning messages in build logs.
@@ -184,8 +200,14 @@ subprojects {
                 name: 'guava',
                 version: guavaVersion,
                 changing: true
+
+        //TODO:2018-05-11:alexander.yevsyukov: Replace the below with Checker Framework
+        // after Guava fully migrates from it too.
         compile "com.google.code.findbugs:jsr305:3.0.0"
 
+        compile "org.checkerframework:checker-qual:$checkerFrameworkVersion"
+        compile "com.google.errorprone:error_prone_annotations:$errorProneVersion"
+
         //As a Library, we provide logging facade API, not specific logger bindings.
         //Target apps are free to use any binding they need.
         compile "org.slf4j:slf4j-api:$slf4JVersion"
@@ -213,12 +235,16 @@ subprojects {
     sourceSets {
         main {
             proto.srcDirs = ["$sourcesRootDir/main/proto"]
-            java.srcDirs = [generatedJavaDir, "$sourcesRootDir/main/java", generatedSpineDir]
+            java.srcDirs = [generatedJavaDir,
+                            "$sourcesRootDir/main/java",
+                            generatedSpineDir]
             resources.srcDirs = ["$generatedRootDir/main/resources"]
         }
         test {
             proto.srcDirs = ["$sourcesRootDir/test/proto"]
-            java.srcDirs = [generatedTestJavaDir, "$sourcesRootDir/test/java", generatedTestSpineDir]
+            java.srcDirs = [generatedTestJavaDir,
+                            "$sourcesRootDir/test/java",
+                            generatedTestSpineDir]
             resources.srcDirs = ["$generatedRootDir/test/resources"]
         }
     }
@@ -290,6 +316,13 @@ subprojects {
         source = sourceSets.main.allJava
     }
 
+    tasks.withType(JavaCompile) {
+        // Exclude generated sources from being analyzed by Error Prone.
+        options.compilerArgs += [
+                '-XepExcludedPaths:.*/generated/.*'
+        ]
+    }
+
     task sourceJar(type: Jar) {
         from sourceSets.main.allJava
         classifier "sources"

client/src/main/java/io/spine/client/ActorRequestFactory.java

@@ -27,8 +27,7 @@
 import io.spine.core.UserId;
 import io.spine.time.ZoneOffset;
 import io.spine.time.ZoneOffsets;
-
-import javax.annotation.Nullable;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 import static com.google.common.base.Preconditions.checkNotNull;
 import static io.spine.base.Time.getCurrentTime;
@@ -53,8 +52,7 @@ public class ActorRequestFactory {
      *
      * <p>This field is null in a single tenant application.
      */
-    @Nullable
-    private final TenantId tenantId;
+    private final @Nullable TenantId tenantId;
 
     protected ActorRequestFactory(Builder builder) {
         this.actor = builder.actor;
@@ -74,8 +72,7 @@ public ZoneOffset getZoneOffset() {
         return zoneOffset;
     }
 
-    @Nullable
-    public TenantId getTenantId() {
+    public @Nullable TenantId getTenantId() {
         return tenantId;
     }
 
@@ -86,10 +83,11 @@ public TenantId getTenantId() {
      * @return new factory at new time zone
      */
     public ActorRequestFactory switchTimezone(ZoneOffset zoneOffset) {
-        final ActorRequestFactory result = newBuilder().setActor(getActor())
-                                                       .setZoneOffset(zoneOffset)
-                                                       .setTenantId(getTenantId())
-                                                       .build();
+        ActorRequestFactory result =
+                newBuilder().setActor(getActor())
+                            .setZoneOffset(zoneOffset)
+                            .setTenantId(getTenantId())
+                            .build();
         return result;
     }
 
@@ -137,11 +135,13 @@ protected CommandContext createCommandContext() {
      * <p>Sets the timestamp value to the
      * {@linkplain io.spine.base.Time#getCurrentTime() current time}.
      */
+    @SuppressWarnings("CheckReturnValue") // calling builder
     ActorContext actorContext() {
-        final ActorContext.Builder builder = ActorContext.newBuilder()
-                                                         .setActor(actor)
-                                                         .setTimestamp(getCurrentTime())
-                                                         .setZoneOffset(zoneOffset);
+        ActorContext.Builder builder = ActorContext
+                .newBuilder()
+                .setActor(actor)
+                .setTimestamp(getCurrentTime())
+                .setZoneOffset(zoneOffset);
         if (tenantId != null) {
             builder.setTenantId(tenantId);
         }
@@ -157,8 +157,7 @@ public static class Builder {
 
         private ZoneOffset zoneOffset;
 
-        @Nullable
-        private TenantId tenantId;
+        private @Nullable TenantId tenantId;
 
         public UserId getActor() {
             return actor;
@@ -169,13 +168,13 @@ public UserId getActor() {
          *
          * @param actor the ID of the user generating commands
          */
+        @CanIgnoreReturnValue
         public Builder setActor(UserId actor) {
             this.actor = checkNotNull(actor);
             return this;
         }
 
-        @Nullable
-        public ZoneOffset getZoneOffset() {
+        public @Nullable ZoneOffset getZoneOffset() {
             return zoneOffset;
         }
 
@@ -184,13 +183,13 @@ public ZoneOffset getZoneOffset() {
          *
          * @param zoneOffset the offset of the timezone the user works in
          */
+        @CanIgnoreReturnValue
         public Builder setZoneOffset(ZoneOffset zoneOffset) {
             this.zoneOffset = checkNotNull(zoneOffset);
             return this;
         }
 
-        @Nullable
-        public TenantId getTenantId() {
+        public @Nullable TenantId getTenantId() {
             return tenantId;
         }
 
@@ -199,6 +198,7 @@ public TenantId getTenantId() {
          *
          * @param tenantId the ID of the tenant or null for single-tenant applications
          */
+        @CanIgnoreReturnValue
         public Builder setTenantId(@Nullable TenantId tenantId) {
             this.tenantId = tenantId;
             return this;
@@ -211,7 +211,6 @@ public Builder setTenantId(@Nullable TenantId tenantId) {
          *
          * @return {@code null}
          */
-        @SuppressWarnings("ReturnOfNull")   // It's fine for an abstract Builder.
         @CanIgnoreReturnValue
         public ActorRequestFactory build() {
             checkNotNull(actor, "`actor` must be defined");