Fix ConstructorThrow SpotBugs warnings in core module (#5785)

* Fix ConstructorThrow SpotBugs warningins in core module

---------

Co-authored-by: Christopher Tubbs <ctubbsii@apache.org>

Author: DomGarguilo

core/pom.xml

@@ -29,6 +29,9 @@
   <artifactId>accumulo-core</artifactId>
   <name>Apache Accumulo Core</name>
   <description>Apache Accumulo core libraries.</description>
+  <properties>
+    <spotbugs.omitVisitors>SharedVariableAtomicityDetector</spotbugs.omitVisitors>
+  </properties>
   <dependencies>
     <dependency>
       <groupId>com.beust</groupId>

core/src/main/java/org/apache/accumulo/core/classloader/URLContextClassLoaderFactory.java

@@ -22,7 +22,6 @@
 import java.net.URL;
 import java.net.URLClassLoader;
 import java.util.Arrays;
-import java.util.concurrent.atomic.AtomicBoolean;
 
 import org.apache.accumulo.core.spi.common.ContextClassLoaderFactory;
 import org.apache.accumulo.core.util.cache.Caches;
@@ -39,9 +38,7 @@
  */
 public class URLContextClassLoaderFactory implements ContextClassLoaderFactory {
 
-  private static final AtomicBoolean isInstantiated = new AtomicBoolean(false);
   private static final Logger LOG = LoggerFactory.getLogger(URLContextClassLoaderFactory.class);
-  private static final String className = URLContextClassLoaderFactory.class.getName();
 
   // Cache the class loaders for re-use
   // WeakReferences are used so that the class loaders can be cleaned up when no longer needed
@@ -50,12 +47,6 @@ public class URLContextClassLoaderFactory implements ContextClassLoaderFactory {
   private final Cache<String,URLClassLoader> classloaders =
       Caches.getInstance().createNewBuilder(CacheName.CLASSLOADERS, true).weakValues().build();
 
-  public URLContextClassLoaderFactory() {
-    if (!isInstantiated.compareAndSet(false, true)) {
-      throw new IllegalStateException("Can only instantiate " + className + " once");
-    }
-  }
-
   @Override
   public ClassLoader getClassLoader(String context) {
     if (context == null) {

core/src/main/java/org/apache/accumulo/core/client/IteratorSetting.java

@@ -51,7 +51,7 @@
  * scanner.addScanIterator(cfg);
  * </pre>
  */
-public class IteratorSetting implements Writable {
+public final class IteratorSetting implements Writable {
   private int priority;
   private String name;
   private String iteratorClass;

core/src/main/java/org/apache/accumulo/core/client/rfile/RFileScanner.java

@@ -77,7 +77,7 @@
 
 import com.google.common.base.Preconditions;
 
-class RFileScanner extends ScannerOptions implements Scanner {
+final class RFileScanner extends ScannerOptions implements Scanner {
 
   private static class RFileScannerEnvironmentImpl extends ClientServiceEnvironmentImpl {
 

core/src/main/java/org/apache/accumulo/core/client/security/tokens/CredentialProviderToken.java

@@ -30,9 +30,13 @@
 import org.apache.accumulo.core.conf.HadoopCredentialProvider;
 import org.apache.hadoop.conf.Configuration;
 
+import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
+
 /**
  * An {@link AuthenticationToken} backed by a Hadoop CredentialProvider.
  */
+@SuppressFBWarnings(value = "CT_CONSTRUCTOR_THROW",
+    justification = "Constructor validation is required for proper initialization")
 public class CredentialProviderToken extends PasswordToken {
   public static final String NAME_PROPERTY = "name";
   public static final String CREDENTIAL_PROVIDERS_PROPERTY = "credentialProviders";

core/src/main/java/org/apache/accumulo/core/client/security/tokens/KerberosToken.java

@@ -34,11 +34,15 @@
 import org.apache.hadoop.security.UserGroupInformation;
 import org.apache.hadoop.security.UserGroupInformation.AuthenticationMethod;
 
+import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
+
 /**
  * Authentication token for Kerberos authenticated clients
  *
  * @since 1.7.0
  */
+@SuppressFBWarnings(value = "CT_CONSTRUCTOR_THROW",
+    justification = "Constructor validation is required for proper initialization")
 public class KerberosToken implements AuthenticationToken {
 
   public static final String CLASS_NAME = KerberosToken.class.getName();

core/src/main/java/org/apache/accumulo/core/client/summary/CounterSummary.java

@@ -33,7 +33,7 @@
  *
  * @since 2.0.0
  */
-public class CounterSummary {
+public final class CounterSummary {
   private Map<String,Long> stats;
 
   private static final Logger log = LoggerFactory.getLogger(CounterSummary.class);

core/src/main/java/org/apache/accumulo/core/clientImpl/ActiveScanImpl.java

@@ -42,7 +42,7 @@
  *
  * @since 1.6.0
  */
-public class ActiveScanImpl extends ActiveScan {
+public final class ActiveScanImpl extends ActiveScan {
 
   private final long scanId;
   private final String client;

core/src/main/java/org/apache/accumulo/core/clientImpl/DelegationTokenImpl.java

@@ -32,7 +32,7 @@
 import org.apache.hadoop.io.Text;
 import org.apache.hadoop.security.token.Token;
 
-public class DelegationTokenImpl extends PasswordToken implements DelegationToken {
+public final class DelegationTokenImpl extends PasswordToken implements DelegationToken {
 
   public static final String SERVICE_NAME = "AccumuloDelegationToken";
 

core/src/main/java/org/apache/accumulo/core/clientImpl/OfflineIterator.java

@@ -72,7 +72,7 @@
 import org.apache.hadoop.fs.FileSystem;
 import org.apache.hadoop.io.Text;
 
-class OfflineIterator implements Iterator<Entry<Key,Value>> {
+final class OfflineIterator implements Iterator<Entry<Key,Value>> {
 
   private SortedKeyValueIterator<Key,Value> iter;
   private Range range;