[FLINK-11867][datastream] Fixed preconditions for filePath's value

flink-streaming-java/src/main/java/org/apache/flink/streaming/api/environment/StreamExecutionEnvironment.java

@@ -77,6 +77,7 @@
 import org.apache.flink.streaming.api.transformations.StreamTransformation;
 import org.apache.flink.util.Preconditions;
 import org.apache.flink.util.SplittableIterator;
+import org.apache.flink.util.StringUtils;
 
 import com.esotericsoftware.kryo.Serializer;
 
@@ -958,8 +959,7 @@ public DataStreamSource<String> readTextFile(String filePath) {
  * @return The data stream that represents the data read from the given file as text lines
  */
  public DataStreamSource<String> readTextFile(String filePath, String charsetName) {
- Preconditions.checkNotNull(filePath, "The file path must not be null.");
- Preconditions.checkNotNull(filePath.isEmpty(), "The file path must not be empty.");
+ Preconditions.checkArgument(!StringUtils.isNullOrWhitespaceOnly(filePath), "The file path must not be null or blank.");
 
  TextInputFormat format = new TextInputFormat(new Path(filePath));
  format.setFilesFilter(FilePathFilter.createDefaultFilter());
@@ -1156,8 +1156,7 @@ public <OUT> DataStreamSource<OUT> readFile(FileInputFormat<OUT> inputFormat,
  TypeInformation<OUT> typeInformation) {
 
  Preconditions.checkNotNull(inputFormat, "InputFormat must not be null.");
- Preconditions.checkNotNull(filePath, "The file path must not be null.");
- Preconditions.checkNotNull(filePath.isEmpty(), "The file path must not be empty.");
+ Preconditions.checkArgument(!StringUtils.isNullOrWhitespaceOnly(filePath), "The file path must not be null or blank.");
 
  inputFormat.setFilePath(filePath);
  return createFileInput(inputFormat, typeInformation, "Custom File Source", watchType, interval);