<input id="treeId" name="treeId" type="hidden" th:value="${dept?.deptId?:''}"/>
	<input id="treeName" name="treeName" type="hidden" th:value="${dept?.deptName?:''}"/>

• ${dept?.deptId}： Use safe operators , Prevent throwing NullPointerException
• ${dept?.deptId?:''}：deptId by null when , Output ''

spring el The expression sets the default value

Reference resources here .

When no default value is set ,deptId by null when , Will be output null.

	<input id="treeId" name="treeId" type="hidden" th:value="${dept.deptId}"/>

After setting the default value ,deptId by null when ,, Will be output ''.

	<input id="treeId" name="treeId" type="hidden" th:value="${dept.deptId?:''}"/>

spring el Expression security operator

Reference resources here .

After using the security operator , Don't worry about NullPointerException 了 .

When there is no safe operator ,dept Variable is null when ,${dept.deptId} Will throw out NullPointerException .

	<input id="treeId" name="treeId" type="hidden" th:value="${dept.deptId}"/>

After using the safe operator ,dept Variable is null when ,${dept?.deptId} And will not throw NullPointerException .

	<input id="treeId" name="treeId" type="hidden" th:value="${dept?.deptId}"/>