利用cors，实现js跨域访问Tomcat下资源

第一步：页面js代码：

function createCORSRequest(method, url){var xhr = new XMLHttpRequest();if ("withCredentials" in xhr){xhr.open(method, url, true);} else if (typeof XDomainRequest != "undefined"){xhr = new XDomainRequest();xhr.open(method, url);} else {xhr = null;}return xhr;
}var request = createCORSRequest("get", "http://192.168.5.221:8080/");
if (request){request.onreadystatechange = function(){if (request.readyState == 4 && request.status == 200) {var response = request.responseText;console.log(response)}   request.send();
}

第二步：在tomcat服务器下webapps/ROOT目录下创建如下两个xml文件
clientaccesspolicy.xml

<?xml version="1.0" encoding="utf-8" ?>
<access-policy><cross-domain-access><policy>  <allow-from http-request-headers="*">  <domain uri="*"/>  </allow-from>  <grant-to>  <resource path="/" include-subpaths="true"/>  </grant-to>  </policy>  </cross-domain-access>  
</access-policy>

crossdomain.xml

<?xml version="1.0"?>  <cross-domain-policy>  <allow-access-from domain="*" />  </cross-domain-policy>

第三步：在Tomcat服务器下conf/web.xml中，或者项目WEB-INF/web.xml中，我选择的是在项目下配置过滤器。我验证了两个，一个是tomcat下自带的cors过滤器，一个是cors-filter-1.7.jar下的过滤器。两个我都测试了，没问题。

应用tomcat的filter配置如下：参数cors.allowOrigin可以指定具体的源来访问

<filter><filter-name>CorsFilter</filter-name><filter-class>org.apache.catalina.filters.CorsFilter</filter-class><init-param><param-name>cors.allowed.origins</param-name><param-value>*</param-value></init-param><init-param><param-name>cors.allowed.methods</param-name><param-value>GET,POST,HEAD,OPTIONS,PUT</param-value></init-param><init-param><param-name>cors.allowed.headers</param-name><param-value>Content-Type,X-Requested-With,accept,Origin,Access-Control-Request-Method,Access-Control-Request-Headers</param-value></init-param><init-param><param-name>cors.exposed.headers</param-name><param-value>Access-Control-Allow-Origin,Access-Control-Allow-Credentials</param-value></init-param><init-param><param-name>cors.support.credentials</param-name><param-value>true</param-value></init-param><init-param><param-name>cors.preflight.maxage</param-name><param-value>10</param-value></init-param>
</filter>
<filter-mapping><filter-name>CorsFilter</filter-name><url-pattern>/*</url-pattern>
</filter-mapping>

或者应用cors-filter-1.7.jar，还需要java-property-utils-1.9.jar，可以在http://mvnrepository.com/artifact/com.thetransactioncompany/cors-filter 选择某个版本导入pom.xml中，自动导入jar包。

配置如下：参数cors.allowOrigin可以指定具体的源来访问

<filter><filter-name>CORS</filter-name><filter-class>com.thetransactioncompany.cors.CORSFilter</filter-class><init-param><param-name>cors.allowOrigin</param-name><param-value>*</param-value></init-param><init-param><param-name>cors.supportedMethods</param-name><param-value>GET, POST, HEAD, PUT, DELETE</param-value></init-param><init-param><param-name>cors.supportedHeaders</param-name><param-value>Accept, Origin, X-Requested-With, Content-Type, Last-Modified</param-value></init-param><init-param><param-name>cors.exposedHeaders</param-name><param-value>Set-Cookie</param-value></init-param><init-param><param-name>cors.supportsCredentials</param-name><param-value>true</param-value></init-param>
</filter>
<filter-mapping><filter-name>CORS</filter-name><url-pattern>/*</url-pattern>
</filter-mapping>