在GWT和Google App Engine中获取客户端IP地址的Java代码

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在GWT和Google App Engine中获取客户端IP地址的Java代码如下：

```java
import javax.servlet.http.HttpServletRequest;

public class IpAddressUtil {

    public static String getIpAddress(HttpServletRequest request) {
        String ipAddress = request.getHeader("X-Forwarded-For");
        if (ipAddress == null || ipAddress.length() == 0 || "unknown".equalsIgnoreCase(ipAddress)) {
            ipAddress = request.getHeader("Proxy-Client-IP");
        }
        if (ipAddress == null || ipAddress.length() == 0 || "unknown".equalsIgnoreCase(ipAddress)) {
            ipAddress = request.getHeader("WL-Proxy-Client-IP");
        }
        if (ipAddress == null || ipAddress.length() == 0 || "unknown".equalsIgnoreCase(ipAddress)) {
            ipAddress = request.getHeader("HTTP_CLIENT_IP");
        }
        if (ipAddress == null || ipAddress.length() == 0 || "unknown".equalsIgnoreCase(ipAddress)) {
            ipAddress = request.getHeader("HTTP_X_FORWARDED_FOR");
        }
        if (ipAddress == null || ipAddress.length() == 0 || "unknown".equalsIgnoreCase(ipAddress)) {
            ipAddress = request.getRemoteAddr();
        }
        return ipAddress;
    }
}
```

这个工具类尝试从多个HTTP请求头中获取客户端IP地址，如果都无法获取到，则返回远程地址。在GWT和Google App Engine中，可以通过HttpServletRequest对象获取客户端IP地址。