Deserialize attributes lazily in JdbcOperationsSessionRepository

Instead of deserializing all of the session attributes as they are read from the database, deserialize as #getAttribute requests them.

See: #1133

spring-session-jdbc/src/main/java/org/springframework/session/jdbc/JdbcOperationsSessionRepository.java

@@ -28,6 +28,7 @@ import java.util.List;
 import java.util.Map;
 import java.util.Set;
 import java.util.UUID;
+import java.util.function.Supplier;
 import java.util.stream.Collectors;
 
 import org.apache.commons.logging.Log;
@@ -530,7 +531,7 @@ public class JdbcOperationsSessionRepository implements
 						public void setValues(PreparedStatement ps, int i) throws SQLException {
 							String attributeName = attributeNames.get(i);
 							ps.setString(1, attributeName);
-							serialize(ps, 2, session.getAttribute(attributeName));
+							bytesToLob(ps, 2, serialize(session.getAttribute(attributeName)));
 							ps.setString(3, session.getId());
 						}
 
@@ -545,7 +546,7 @@ public class JdbcOperationsSessionRepository implements
 			this.jdbcOperations.update(this.createSessionAttributeQuery, (ps) -> {
 				String attributeName = attributeNames.get(0);
 				ps.setString(1, attributeName);
-				serialize(ps, 2, session.getAttribute(attributeName));
+				bytesToLob(ps, 2, serialize(session.getAttribute(attributeName)));
 				ps.setString(3, session.getId());
 			});
 		}
@@ -559,7 +560,7 @@ public class JdbcOperationsSessionRepository implements
 						@Override
 						public void setValues(PreparedStatement ps, int i) throws SQLException {
 							String attributeName = attributeNames.get(i);
-							serialize(ps, 1, session.getAttribute(attributeName));
+							bytesToLob(ps, 1, serialize(session.getAttribute(attributeName)));
 							ps.setString(2, session.primaryKey);
 							ps.setString(3, attributeName);
 						}
@@ -574,7 +575,7 @@ public class JdbcOperationsSessionRepository implements
 		else {
 			this.jdbcOperations.update(this.updateSessionAttributeQuery, (ps) -> {
 				String attributeName = attributeNames.get(0);
-				serialize(ps, 1, session.getAttribute(attributeName));
+				bytesToLob(ps, 1, serialize(session.getAttribute(attributeName)));
 				ps.setString(2, session.primaryKey);
 				ps.setString(3, attributeName);
 			});
@@ -657,18 +658,26 @@ public class JdbcOperationsSessionRepository implements
 				getQuery(DELETE_SESSIONS_BY_EXPIRY_TIME_QUERY);
 	}
 
-	private void serialize(PreparedStatement ps, int paramIndex, Object attributeValue)
+	private void bytesToLob(PreparedStatement ps, int paramIndex, byte[] bytes)
 			throws SQLException {
 		this.lobHandler.getLobCreator().setBlobAsBytes(ps, paramIndex,
-				(byte[]) this.conversionService.convert(attributeValue,
-						TypeDescriptor.valueOf(Object.class),
-						TypeDescriptor.valueOf(byte[].class)));
+				bytes);
 	}
 
-	private Object deserialize(ResultSet rs, String columnName)
+	private byte[] serialize(Object attributeValue) {
+		return (byte[]) this.conversionService.convert(attributeValue,
+						TypeDescriptor.valueOf(Object.class),
+						TypeDescriptor.valueOf(byte[].class));
+	}
+
+	private byte[] lobToBytes(ResultSet rs, String columnName)
 			throws SQLException {
+		return this.lobHandler.getBlobAsBytes(rs, columnName);
+	}
+
+	private Object deserialize(byte[] bytes) {
 		return this.conversionService.convert(
-				this.lobHandler.getBlobAsBytes(rs, columnName),
+				bytes,
 				TypeDescriptor.valueOf(byte[].class),
 				TypeDescriptor.valueOf(Object.class));
 	}
@@ -679,6 +688,34 @@ public class JdbcOperationsSessionRepository implements
 
 	}
 
+	static final <Z> Supplier<Z> constantSupplier(Z value) {
+		if (value == null) {
+			return null;
+		}
+		else {
+			return () -> value;
+		}
+	}
+
+	static final <Z> Supplier<Z> lazily(Supplier<Z> supplier) {
+		if (supplier == null) {
+			return null;
+		}
+		else {
+			return new Supplier<Z>() {
+				private Z value;
+
+				@Override
+				public Z get() {
+					if (this.value == null) {
+						this.value = supplier.get();
+					}
+					return this.value;
+				}
+			};
+		}
+	}
+
 	/**
 	 * The {@link Session} to use for {@link JdbcOperationsSessionRepository}.
 	 *
@@ -748,7 +785,13 @@ public class JdbcOperationsSessionRepository implements
 
 		@Override
 		public <T> T getAttribute(String attributeName) {
-			return this.delegate.getAttribute(attributeName);
+			Supplier<T> supplier = this.delegate.getAttribute(attributeName);
+			if (supplier == null) {
+				return null;
+			}
+			else {
+				return supplier.get();
+			}
 		}
 
 		@Override
@@ -783,7 +826,7 @@ public class JdbcOperationsSessionRepository implements
 								? oldDeltaValue
 								: DeltaValue.UPDATED);
 			}
-			this.delegate.setAttribute(attributeName, attributeValue);
+			this.delegate.setAttribute(attributeName, constantSupplier(attributeValue));
 			if (PRINCIPAL_NAME_INDEX_NAME.equals(attributeName) ||
 					SPRING_SECURITY_CONTEXT.equals(attributeName)) {
 				this.changed = true;
@@ -875,7 +918,8 @@ public class JdbcOperationsSessionRepository implements
 				}
 				String attributeName = rs.getString("ATTRIBUTE_NAME");
 				if (attributeName != null) {
-					session.delegate.setAttribute(attributeName, deserialize(rs, "ATTRIBUTE_BYTES"));
+					byte[] bytes = lobToBytes(rs, "ATTRIBUTE_BYTES");
+					session.delegate.setAttribute(attributeName, lazily(() -> deserialize(bytes)));
 				}
 				sessions.add(session);
 			}