refactor: Use an empty collection instead of null return value

apollo-audit/apollo-audit-impl/src/main/java/com/ctrip/framework/apollo/audit/component/ApolloAuditLogApiNoOpImpl.java

@@ -21,6 +21,7 @@
 import com.ctrip.framework.apollo.audit.dto.ApolloAuditLogDataInfluenceDTO;
 import com.ctrip.framework.apollo.audit.dto.ApolloAuditLogDetailsDTO;
 import com.ctrip.framework.apollo.audit.dto.ApolloAuditLogDTO;
+import java.util.Collections;
 import java.util.Date;
 import java.util.List;
 
@@ -50,28 +51,28 @@ public void appendDataInfluences(List<Object> entities, Class<?> beanDefinition)
 
   @Override
   public List<ApolloAuditLogDTO> queryLogs(int page, int size) {
-    return null;
+    return Collections.emptyList();
   }
 
   @Override
   public List<ApolloAuditLogDTO> queryLogsByOpName(String opName, Date startDate,
       Date endDate, int page, int size) {
-    return null;
+    return Collections.emptyList();
   }
 
   @Override
   public List<ApolloAuditLogDetailsDTO> queryTraceDetails(String traceId) {
-    return null;
+    return Collections.emptyList();
   }
 
   @Override
   public List<ApolloAuditLogDataInfluenceDTO> queryDataInfluencesByField(String entityName,
       String entityId, String fieldName, int page, int size) {
-    return null;
+    return Collections.emptyList();
   }
 
   @Override
   public List<ApolloAuditLogDTO> searchLogByNameOrTypeOrOperator(String query, int page, int size) {
-    return null;
+    return Collections.emptyList();
   }
 }

apollo-biz/src/main/java/com/ctrip/framework/apollo/biz/utils/ReleaseMessageKeyGenerator.java

@@ -21,6 +21,7 @@
 
 import com.ctrip.framework.apollo.core.ConfigConsts;
 import com.google.common.base.Splitter;
+import java.util.Collections;
 import java.util.List;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -42,7 +43,7 @@ public static List<String> messageToList(String message) {
     //message should be appId+cluster+namespace
     if (keys.size() != 3) {
       logger.error("message format invalid - {}", message);
-      return null;
+      return Collections.emptyList();
     }
     return keys;
   }

apollo-common/src/main/java/com/ctrip/framework/apollo/common/datasource/ApolloDataSourceScriptDatabaseInitializerFactory.java

@@ -20,6 +20,7 @@
 import java.security.CodeSource;
 import java.util.ArrayList;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.List;
 import javax.sql.DataSource;
 import org.springframework.boot.jdbc.DataSourceBuilder;
@@ -101,7 +102,7 @@ private static List<String> resolveLocations(Collection<String> locations,
   private static Collection<String> convertRepositoryLocations(Collection<String> locations,
       DataSource dataSource) {
     if (CollectionUtils.isEmpty(locations)) {
-      return null;
+      return Collections.emptyList();
     }
     String repositoryDir = findRepositoryDirectory();
     String suffix = findSuffix(dataSource);

apollo-portal/src/main/java/com/ctrip/framework/apollo/portal/spi/defaultimpl/DefaultRolePermissionService.java

@@ -183,7 +183,7 @@ public Set<UserInfo> queryUsersWithRole(String roleName) {
         List<UserRole> userRoles = userRoleRepository.findByRoleId(role.getId());
         List<UserInfo> userInfos = userService.findByUserIds(userRoles.stream().map(UserRole::getUserId).collect(Collectors.toList()));
 
-        if(userInfos == null){
+        if (CollectionUtils.isEmpty(userInfos)) {
             return Collections.emptySet();
         }
 

apollo-portal/src/main/java/com/ctrip/framework/apollo/portal/spi/defaultimpl/DefaultUserService.java

@@ -24,37 +24,44 @@
 import java.util.Collections;
 import java.util.List;
 import java.util.Objects;
+import org.springframework.util.CollectionUtils;
 
 /**
  * @author Jason Song([email protected])
  */
 public class DefaultUserService implements UserService {
 
+  private static final String DEFAULT_USER_ID = "apollo";
+
   @Override
   public List<UserInfo> searchUsers(String keyword, int offset, int limit, boolean includeInactiveUsers) {
     return Collections.singletonList(assembleDefaultUser());
   }
 
   @Override
   public UserInfo findByUserId(String userId) {
-    if (Objects.equals(userId, "apollo")) {
+    if (Objects.equals(userId, DEFAULT_USER_ID)) {
       return assembleDefaultUser();
     }
     return null;
   }
 
   @Override
   public List<UserInfo> findByUserIds(List<String> userIds) {
-    if (userIds.contains("apollo")) {
+    if (CollectionUtils.isEmpty(userIds)) {
+      return Collections.emptyList();
+    }
+
+    if (userIds.contains(DEFAULT_USER_ID)) {
       return Lists.newArrayList(assembleDefaultUser());
     }
-    return null;
+    return Collections.emptyList();
   }
 
   private UserInfo assembleDefaultUser() {
     UserInfo defaultUser = new UserInfo();
-    defaultUser.setUserId("apollo");
-    defaultUser.setName("apollo");
+    defaultUser.setUserId(DEFAULT_USER_ID);
+    defaultUser.setName(DEFAULT_USER_ID);
     defaultUser.setEmail("[email protected]");
 
     return defaultUser;