Project facet Dynamic Web 3.1

If you want to use servlet-api on version 3.1 you need to use the following schema:

• http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd

Note that 3.0 and 3.1 are different: in 3.1 there's no Sun mentioned, so simply changing 3_0.xsd to 3_1.xsd won't work

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee 
                http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
         version="3.1" xmlns="http://xmlns.jcp.org/xml/ns/javaee">
</web-app>

Also, make sure you're depending on the latest versions in your pom.xml. That is,

<plugin>
    <artifactId>maven-war-plugin</artifactId>
    <version>2.6</version>
    <configuration>
        <failOnMissingWebXml>false</failOnMissingWebXml>
    </configuration>
</plugin>

and

<dependency>
    <groupId>javax.servlet</groupId>
    <artifactId>javax.servlet-api</artifactId>
    <version>3.1.0</version>
    <scope>provided</scope>
</dependency>

or

<dependency>
    <groupId>javax</groupId>
    <artifactId>javaee-api</artifactId>
    <version>7.0</version>
</dependency>

Also, you should compile with Java 7 or 8:

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <version>3.3</version>
    <configuration>
        <source>1.8</source><!--源代码使用的开发版本 -->
        <target>1.8</target><！-- 需要生成的目标class文件的编译版本 -->
        <!-- 一般而言，target与source是保持一致的，但是，有时候为了让程序能在其他版本的jdk中运行(对于低版本目标jdk，源代码中需要没有使用低版本jdk中不支持的语法)，会存在target不同于source的情况 -->
     
        <!-- 这下面的是可选项 -->
        <meminitial>128m</meminitial>
        <maxmem>512m</maxmem>
        <fork>true</fork> <!-- fork is enable,用于明确表示编译版本配置的可用 --> 
        <compilerVersion>1.3</compilerVersion>
        <!-- 这个选项用来传递编译器自身不包含但是却支持的参数选项 -->
        <compilerArgument>-verbose -bootclasspath ${java.home}\lib\rt.jar</compilerArgument>
    </configuration>
</plugin>