在virtual server为81端口的实例中添加如下内容：

location ~ .*\.(gif|jpg|jpeg|png|bmp|swf)$
 {
  root /webapp/tomcat7/webapps/sdws;
  if (-f $request_filename) {
   expires 1d;
   break;
   }
  }
    location ~ .*\.(js|css)$
 {
  root /webapp/tomcat7/webapps/;
  if (-f $request_filename) {
   expires 1d;
   break;
   }
} //by http://www.it.net.cn

重启nginx或重新加载配置即可生效。

附，本问题用到的nginx.conf配置文件的完整内容。

#user  nobody;

worker_processes  1;

error_log  logs/error.log;
#error_log  logs/error.log  notice;
#error_log  logs/error.log  info;
#pid logs/nginx.pid;

events {
    worker_connections  1024;
}

http {
    include       mime.types;
    default_type  application/octet-stream;
   #log_format  main  '$remote_addr - $remote_user [$time_local] "$request" '
   #    '$status $body_bytes_sent "$http_referer" '
   #    '"$http_user_agent" "$http_x_forwarded_for"';
   #access_log  logs/access.log  main;
   sendfile on;

   #tcp_nopush     on;
   #keepalive_timeout  0;
   keepalive_timeout  65;
   #gzip  on;

server {
#实例其中之一
 listen       81;
 server_name  10.0.211.5;
 #charset koi8-r;
 #access_log  logs/host.access.log  main;

 location / {
        proxy_pass http://127.0.0.1:8081/sdws/;
        proxy_set_header Host    $host;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
 }

 #error_page  404       /404.html;
 # redirect server error pages to the static page /50x.html
 #

 error_page   500 502 503 504  /50x.html;
 location = /50x.html {
     root   html;
 }

 location ~ .*\.(gif|jpg|jpeg|png|bmp|swf)$
 {
  root /webapp/tomcat7/webapps/sdws;
  if (-f $request_filename) {
   expires 1d;
   break;
   }
  }

location ~ .*\.(js|css)$
 {
  root /webapp/tomcat7/webapps/;
  if (-f $request_filename) {
   expires 1d;
   break;
   }
  } //by http://www.it.net.cn

 # proxy the PHP scripts to Apache listening on 127.0.0.1:80
 #
 #location ~ \.php$ {
 #    proxy_pass   http://127.0.0.1;
 #}

 # pass the PHP scripts to FastCGI server listening on 127.0.0.1:9000
 #

 #location ~ \.php$ {
 #    root    html;
 #    fastcgi_pass   127.0.0.1:9000;
 #    fastcgi_index  index.php;
 #    fastcgi_param  SCRIPT_FILENAME  /scripts$fastcgi_script_name;
 #    include fastcgi_params;
 #}
 # deny access to .htaccess files, if Apache's document root
 # concurs with nginx's one
 #
 #location ~ /\.ht {
 #    deny  all;
 #}
    }
    # another virtual host using mix of IP-, name-, and port-based configuration
    #
    #server {
    #    listen       8000;
    #    listen       somename:8080;
    #    server_name  somename  alias  another.alias;
    #    location / {
   # root   html;
    # index  index.html index.htm;
    #    }
    #}
    # HTTPS server
    #
    #server {
    #    listen       443;
    #    server_name  localhost;
    #    ssl    on;
    #    ssl_certificate      cert.pem;
    #    ssl_certificate_key  cert.key;
    #    ssl_session_timeout  5m;

    #    ssl_protocols  SSLv2 SSLv3 TLSv1;

    #    ssl_ciphers  HIGH:!aNULL:!MD5;

    #    ssl_prefer_server_ciphers   on;

    #    location / {

    # root   html;

    # index  index.html index.htm;

    #    }

    #}
}

gzip_types application/x-javascript text/css