[build] Enable spotbugs (#3601)

Benign spotbugs warnings were suppressed, and all others were fixed. Bug
descriptions are documented here:
https://spotbugs.readthedocs.io/en/stable/bugDescriptions.html

Co-authored-by: Austin Shalit <austinshalit@gmail.com>

shared/java/javacommon.gradle

@@ -1,6 +1,5 @@
 apply plugin: 'maven-publish'
 apply plugin: 'java-library'
-//apply plugin: 'net.ltgt.errorprone'
 apply plugin: 'jacoco'
 
 def baseArtifactId = project.baseId
@@ -110,9 +109,6 @@ dependencies {
     testRuntimeOnly 'org.junit.jupiter:junit-jupiter-engine:5.4.2'
 
     devImplementation sourceSets.main.output
-
-    //errorprone 'com.google.errorprone:error_prone_core:2.3.2-SNAPSHOT'
-    //errorproneJavac 'com.google.errorprone:error_prone_core:2.3.1'
 }
 
 task run(type: JavaExec) {

shared/java/javastyle.gradle

@@ -16,9 +16,63 @@ if (!project.hasProperty('skipJavaFormat')) {
         ruleSetFiles = files(new File(rootDir, "styleguide/pmd-ruleset.xml"))
         ruleSets = []
     }
+
+    apply plugin: 'com.diffplug.spotless'
+
+    spotless {
+        java {
+            target fileTree('.') {
+                include '**/*.java'
+                exclude '**/build/**', '**/build-*/**'
+            }
+            toggleOffOn()
+            googleJavaFormat()
+            removeUnusedImports()
+            trimTrailingWhitespace()
+            endWithNewline()
+        }
+        groovyGradle {
+            target fileTree('.') {
+                include '**/*.gradle'
+                exclude '**/build/**', '**/build-*/**'
+            }
+            greclipse()
+            indentWithSpaces(4)
+            trimTrailingWhitespace()
+            endWithNewline()
+        }
+        format 'xml', {
+            target fileTree('.') {
+                include '**/*.xml'
+                exclude '**/build/**', '**/build-*/**'
+            }
+            eclipseWtp('xml')
+            trimTrailingWhitespace()
+            indentWithSpaces(2)
+            endWithNewline()
+        }
+        format 'misc', {
+            target fileTree('.') {
+                include '**/*.md', '**/.gitignore'
+                exclude '**/build/**', '**/build-*/**'
+            }
+            trimTrailingWhitespace()
+            indentWithSpaces(2)
+            endWithNewline()
+        }
+    }
+
+    apply plugin: 'com.github.spotbugs'
+
+    spotbugs {
+        ignoreFailures = false
+        effort = 'max'
+        excludeFilter = file("${project.rootDir}/styleguide/spotbugs-exclude.xml")
+    }
 }
 
 task javaFormat {
     dependsOn(tasks.withType(Checkstyle))
     dependsOn(tasks.withType(Pmd))
 }
+javaFormat.dependsOn 'spotlessApply'