[FLINK-11744][core]Provide stable/final toHexString-method in AbstractID

This closes apache#7825.

flink-core/src/main/java/org/apache/flink/util/AbstractID.java

@@ -47,7 +47,7 @@ public class AbstractID implements Comparable<AbstractID>, java.io.Serializable
 	protected final long lowerPart;
 
 	/** The memoized value returned by toString(). */
-	private transient String toString;
+	private transient String hexString;
 
 	// --------------------------------------------------------------------------------------------
 
@@ -127,6 +127,22 @@ public byte[] getBytes() {
 		return bytes;
 	}
 
+	/**
+	 * Returns pure String representation of the ID in hexadecimal. This method should be used to construct things like
+	 * paths etc., that require a stable representation and is therefore final.
+	 */
+	public final String toHexString() {
+		if (this.hexString == null) {
+			final byte[] ba = new byte[SIZE];
+			longToByteArray(this.lowerPart, ba, 0);
+			longToByteArray(this.upperPart, ba, SIZE_OF_LONG);
+
+			this.hexString = StringUtils.byteToHexString(ba);
+		}
+
+		return this.hexString;
+	}
+
 	// --------------------------------------------------------------------------------------------
 	//  Standard Utilities
 	// --------------------------------------------------------------------------------------------
@@ -153,15 +169,7 @@ public int hashCode() {
 
 	@Override
 	public String toString() {
-		if (this.toString == null) {
-			final byte[] ba = new byte[SIZE];
-			longToByteArray(this.lowerPart, ba, 0);
-			longToByteArray(this.upperPart, ba, SIZE_OF_LONG);
-
-			this.toString = StringUtils.byteToHexString(ba);
-		}
-
-		return this.toString;
+		return toHexString();
 	}
 
 	@Override

flink-runtime/src/main/java/org/apache/flink/runtime/state/TaskExecutorLocalStateStoresManager.java

@@ -120,7 +120,7 @@ public TaskLocalStateStore localStateStoreForSubtask(
 
 				if (LOG.isDebugEnabled()) {
 					LOG.debug("Registered new allocation id {} for local state stores for job {}.",
-						allocationID, jobId);
+						allocationID.toHexString(), jobId);
 				}
 			}
 
@@ -232,7 +232,7 @@ File[] getLocalStateRootDirectories() {
 
 	@VisibleForTesting
 	String allocationSubDirString(AllocationID allocationID) {
-		return "aid_" + allocationID;
+		return "aid_" + allocationID.toHexString();
 	}
 
 	private File[] allocationBaseDirectories(AllocationID allocationID) {

flink-runtime/src/main/java/org/apache/flink/runtime/state/TaskLocalStateStoreImpl.java

@@ -357,7 +357,7 @@ public String toString() {
 		return "TaskLocalStateStore{" +
 			"jobID=" + jobID +
 			", jobVertexID=" + jobVertexID +
-			", allocationID=" + allocationID +
+			", allocationID=" + allocationID.toHexString() +
 			", subtaskIndex=" + subtaskIndex +
 			", localRecoveryConfig=" + localRecoveryConfig +
 			", storedCheckpointIDs=" + storedTaskStateByCheckpointID.keySet() +