把代码导入eclipse后，web.xml中有这样的2个报错。
cvc-elt.1.a: Cannot find the declaration of element 'web-app'.
Multiple annotations found at this line:
- schema_reference.4: Failed to read schema document 'http://java.sun.com/xml/ns/javaee/
web-app_2_5.xsd', because 1) could not find the document; 2) the document could not be read; 3) the
root element of the document is not xsd:schema. - There are '2' errors in 'http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd'.
