解決步驟:

1.開啟web.xml檔案

把頭部中2.3

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_3.xsd"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_3.xsd"
    id="WebApp_ID" version="2.3">

改為3.0

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
    id="WebApp_ID" version="3.0">

2.關閉eclipse   開啟專案.settings目錄下的org.eclipse.jdt.core.prefs

eclipse.preferences.version=1
org.eclipse.jdt.core.compiler.codegen.inlineJsrBytecode=enabled
org.eclipse.jdt.core.compiler.codegen.targetPlatform=1.5
org.eclipse.jdt.core.compiler.compliance=1.5
org.eclipse.jdt.core.compiler.problem.assertIdentifier=error
org.eclipse.jdt.core.compiler.problem.enumIdentifier=error
org.eclipse.jdt.core.compiler.problem.forbiddenReference=warning
org.eclipse.jdt.core.compiler.source=1.5

把1.5改為1.7

eclipse.preferences.version=1
org.eclipse.jdt.core.compiler.codegen.inlineJsrBytecode=enabled
org.eclipse.jdt.core.compiler.codegen.targetPlatform=1.7
org.eclipse.jdt.core.compiler.compliance=1.7
org.eclipse.jdt.core.compiler.problem.assertIdentifier=error
org.eclipse.jdt.core.compiler.problem.enumIdentifier=error
org.eclipse.jdt.core.compiler.problem.forbiddenReference=warning
org.eclipse.jdt.core.compiler.source=1.7

3.開啟org.eclipse.wst.common.component

<?xml version="1.0" encoding="UTF-8"?>
<project-modules id="moduleCoreId" project-version="1.5.0">
    <wb-module deploy-name="test">
        <wb-resource deploy-path="/" source-path="/target/m2e-wtp/web-resources"/>
        <wb-resource deploy-path="/" source-path="/src/main/webapp" tag="defaultRootSource"/>
        <wb-resource deploy-path="/WEB-INF/classes" source-path="/src/main/java"/>
        <wb-resource deploy-path="/WEB-INF/classes" source-path="/src/main/resources"/>
        <property name="context-root" value="test"/>
        <property name="java-output-path" value="/test/target/classes"/>
    </wb-module>
</project-modules>

將1.5.0改為1.7.0

<?xml version="1.0" encoding="UTF-8"?>
<project-modules id="moduleCoreId" project-version="1.7.0">
    <wb-module deploy-name="test">
        <wb-resource deploy-path="/" source-path="/target/m2e-wtp/web-resources"/>
        <wb-resource deploy-path="/" source-path="/src/main/webapp" tag="defaultRootSource"/>
        <wb-resource deploy-path="/WEB-INF/classes" source-path="/src/main/java"/>
        <wb-resource deploy-path="/WEB-INF/classes" source-path="/src/main/resources"/>
        <property name="context-root" value="test"/>
        <property name="java-output-path" value="/test/target/classes"/>
    </wb-module>
</project-modules>

4.開啟org.eclipse.wst.common.project.facet.core.xml

<?xml version="1.0" encoding="UTF-8"?>
<faceted-project>
  <fixed facet="wst.jsdt.web"/>
  <installed facet="java" version="1.5"/>
  <installed facet="jst.web" version="2.3"/>
  <installed facet="wst.jsdt.web" version="1.0"/>
</faceted-project>

將1.5改為1.7    2.3改為3.0

<?xml version="1.0" encoding="UTF-8"?>
<faceted-project>
  <fixed facet="wst.jsdt.web"/>
  <installed facet="java" version="1.7"/>
  <installed facet="jst.web" version="3.0"/>
  <installed facet="wst.jsdt.web" version="1.0"/>
</faceted-project>

此時重新開啟eclipse,一般人到這個時候紅xx已經解決了,但是LZ開啟一看紅x竟然還在,心拔涼拔涼的,沒辦法,生活還是要繼續,於是LZ又繼續找啊找,又被我找到個方法

5.在pom.xml中增加一段程式碼

　　　　<plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.3</version>
                <configuration>
                    <!-- 指定source和target的版本 -->
                    <source>1.7</source>
                    <target>1.7</target>
                </configuration>
            </plugin>
        </plugins>

此時紅x依舊在,哭死,感覺生活沒了希望。最後右擊專案 Maven -> Update Project 紅x居然消失了,真開心,雖然不知道什麼原理,但是先記錄一下,以防以後又出現這種請款。