Add @Nullable annotations where appropriate

Found via error-prone 2.1.0.

pom.xml

@@ -323,6 +323,11 @@
       <artifactId>jackson-dataformat-xml</artifactId>
       <version>2.8.8</version>
     </dependency>
+    <dependency>
+      <groupId>com.google.code.findbugs</groupId>
+      <artifactId>jsr305</artifactId>
+      <version>3.0.2</version>
+    </dependency>
     <dependency>
       <groupId>commons-fileupload</groupId>
       <artifactId>commons-fileupload</artifactId>

src/main/java/org/gaul/s3proxy/AwsSignature.java

@@ -29,6 +29,7 @@ import java.util.List;
 import java.util.Map;
 import java.util.Set;
 
+import javax.annotation.Nullable;
 import javax.crypto.Mac;
 import javax.crypto.spec.SecretKeySpec;
 import javax.servlet.http.HttpServletRequest;
@@ -171,6 +172,7 @@ final class AwsSignature {
         return BaseEncoding.base16().lowerCase().encode(hash);
     }
 
+    @Nullable
     private static String[] extractSignedHeaders(String authorization) {
         int index = authorization.indexOf("SignedHeaders=");
         if (index < 0) {

src/main/java/org/gaul/s3proxy/NullBlobStore.java

@@ -20,6 +20,8 @@ import java.io.IOException;
 import java.io.InputStream;
 import java.util.List;
 
+import javax.annotation.Nullable;
+
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableSet;
 import com.google.common.hash.HashCode;
@@ -53,6 +55,7 @@ final class NullBlobStore extends ForwardingBlobStore {
     }
 
     @Override
+    @Nullable
     public BlobMetadata blobMetadata(String container, String name) {
         Blob blob = getBlob(container, name);
         if (blob == null) {
@@ -62,11 +65,13 @@ final class NullBlobStore extends ForwardingBlobStore {
     }
 
     @Override
+    @Nullable
     public Blob getBlob(String container, String name) {
         return getBlob(container, name, GetOptions.NONE);
     }
 
     @Override
+    @Nullable
     public Blob getBlob(String container, String name, GetOptions options) {
         Blob blob = super.getBlob(container, name, options);
         if (blob == null) {

src/main/java/org/gaul/s3proxy/S3ProxyHandler.java

@@ -46,6 +46,7 @@ import java.util.TreeMap;
 import java.util.TreeSet;
 import java.util.concurrent.TimeUnit;
 
+import javax.annotation.Nullable;
 import javax.crypto.Mac;
 import javax.crypto.spec.SecretKeySpec;
 import javax.servlet.http.HttpServletRequest;
@@ -212,6 +213,7 @@ public class S3ProxyHandler {
         if (authenticationType != AuthenticationType.NONE) {
             anonymousIdentity = false;
             blobStoreLocator = new BlobStoreLocator() {
+                @Nullable
                 @Override
                 public Map.Entry<String, BlobStore> locateBlobStore(
                         String identityArg, String container, String blob) {

src/test/java/org/gaul/s3proxy/AwsSdkTest.java

@@ -32,6 +32,7 @@ import java.util.Map;
 import java.util.Random;
 import java.util.concurrent.TimeUnit;
 
+import javax.annotation.Nullable;
 import javax.net.ssl.HostnameVerifier;
 import javax.net.ssl.HttpsURLConnection;
 import javax.net.ssl.SSLContext;
@@ -1381,6 +1382,7 @@ public final class AwsSdkTest {
     private static final class NullX509TrustManager
             implements X509TrustManager {
         @Override
+        @Nullable
         public X509Certificate[] getAcceptedIssuers() {
             return null;
         }