CSP stands for Content Security Policy.

import java.io.IOException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;
import java.util.ArrayList;
import java.util.List;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebFilter;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.apache.commons.codec.binary.Hex;

/**
 * Sample filter implementation to define a set of Content Security Policies.<br/>
 * This implementation has a dependency on Commons Codec API.<br/>
 * This filter set CSP policies using all HTTP headers defined into W3C specification.<br/>
 * <br/>
 * This implementation is oriented to be easily understandable and easily adapted.<br/>
 */
@WebFilter("/*")
public class CSPPoliciesApplier implements Filter {

    /** Configuration member to specify if web app use web fonts */
    public static final boolean APP_USE_WEBFONTS = false;

    /** Configuration member to specify if web app use videos or audios */
    public static final boolean APP_USE_AUDIOS_OR_VIDEOS = false;

    /**
     * Configuration member to specify if filter must add CSP directive used by Mozilla (Firefox)
     */
    public static final boolean INCLUDE_MOZILLA_CSP_DIRECTIVES = true;

    /** Filter configuration */
    @SuppressWarnings("unused")
    private FilterConfig filterConfig = null;

    /** List CSP HTTP Headers */
    private List<String> cspHeaders = new ArrayList<String>();

    /** Collection of CSP polcies that will be applied */
    private String policies = null;

    /** Used for Script Nonce */
    private SecureRandom prng = null;

    /**
     * Used to prepare (one time for all) set of CSP policies that will be applied on each HTTP
     * response.
     *
     * @see javax.servlet.Filter#init(javax.servlet.FilterConfig)
     */
    @Override
    public void init(FilterConfig fConfig) throws ServletException {
// Get filter configuration
        this.filterConfig = fConfig;

// Init secure random
        try {
            this.prng = SecureRandom.getInstance("SHA1PRNG");
        } catch (NoSuchAlgorithmException e) {
            throw new ServletException(e);
        }

// Define list of CSP HTTP Headers
        this.cspHeaders.add("Content-Security-Policy");
        this.cspHeaders.add("X-Content-Security-Policy");
        this.cspHeaders.add("X-WebKit-CSP");

// Define CSP policies
// Loading policies for Frame and Sandboxing will be dynamically defined : We need to know if context use Frame
        List<String> cspPolicies = new ArrayList<String>();
        String originLocationRef = "'self'";
        // --Disable default source in order to avoid browser fallback loading using 'default-src'
        // locations
        cspPolicies.add("default-src 'none'");
        // --Define loading policies for Scripts
        cspPolicies.add("script-src " + originLocationRef + " 'unsafe-inline' 'unsafe-eval'");
        if (INCLUDE_MOZILLA_CSP_DIRECTIVES) {
            cspPolicies.add("options inline-script eval-script");
            cspPolicies.add("xhr-src 'self'");
        }
        // --Define loading policies for Plugins
        cspPolicies.add("object-src " + originLocationRef);
        // --Define loading policies for Styles (CSS)
        cspPolicies.add("style-src " + originLocationRef);
        // --Define loading policies for Images
        cspPolicies.add("img-src " + originLocationRef);
        // --Define loading policies for Form
        cspPolicies.add("form-action " + originLocationRef);
        // --Define loading policies for Audios/Videos
        if (APP_USE_AUDIOS_OR_VIDEOS) {
            cspPolicies.add("media-src " + originLocationRef);
        }
        // --Define loading policies for Fonts
        if (APP_USE_WEBFONTS) {
            cspPolicies.add("font-src " + originLocationRef);
        }
        // --Define loading policies for Connection
        cspPolicies.add("connect-src " + originLocationRef);
        // --Define loading policies for Plugins Types
        cspPolicies.add("plugin-types application/pdf application/x-shockwave-flash");
        // --Define browser XSS filtering feature running mode
        cspPolicies.add("reflected-xss block");

        // Target formating
        this.policies = cspPolicies.toString().replaceAll("(\\[|\\])", "").replaceAll(",", ";")
                .trim();
    }

    /**
     * Add CSP policies on each HTTP response.
     *
     * @see javax.servlet.Filter#doFilter(javax.servlet.ServletRequest,
     *      javax.servlet.ServletResponse, javax.servlet.FilterChain)
     */
    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain fchain)
            throws IOException, ServletException {
        HttpServletRequest httpRequest = ((HttpServletRequest) request);
        HttpServletResponse httpResponse = ((HttpServletResponse) response);

        /* Step 1 : Detect if target resource is a Frame */
        // Customize here according to your context...
        boolean isFrame = true;

        /* Step 2 : Add CSP policies to HTTP response */
        StringBuilder policiesBuffer = new StringBuilder(this.policies);

        // If resource is a frame add Frame/Sandbox CSP policy
        if (isFrame) {
            // Frame + Sandbox : Here sandbox allow nothing, customize sandbox options depending on
            // your app....
            policiesBuffer.append(";").append("frame-src 'self';sandbox");
            if (INCLUDE_MOZILLA_CSP_DIRECTIVES) {
                policiesBuffer.append(";").append("frame-ancestors 'self'");
            }
        }

        // Add Script Nonce CSP Policy
        // --Generate a random number
        String randomNum = new Integer(this.prng.nextInt()).toString();
        // --Get its digest
        MessageDigest sha;
        try {
            sha = MessageDigest.getInstance("SHA-1");
        } catch (NoSuchAlgorithmException e) {
            throw new ServletException(e);
        }
        byte[] digest = sha.digest(randomNum.getBytes());
        // --Encode it into HEXA
        String scriptNonce = Hex.encodeHexString(digest);
        policiesBuffer.append(";").append("script-nonce ").append(scriptNonce);
        // --Made available script nonce in view app layer
        httpRequest.setAttribute("CSP_SCRIPT_NONCE", scriptNonce);

        // Add policies to all HTTP headers
        for (String header : this.cspHeaders) {
            httpResponse.setHeader(header, policiesBuffer.toString());
        }

        /* Step 3 : Let request continue chain filter */
        fchain.doFilter(request, response);
    }

    /**
     * {@inheritDoc}
     *
     * @see javax.servlet.Filter#destroy()
     */
    @Override
    public void destroy() {
        // Not used
    }
}