This is the suitable tool for your problem - BrowserMobProxy. With this, you could capture the traffic as a Har file.
You could also block and bypass any request after analyzing the response to decrease the page load time for your automated testing services (if you are doing web automation). It is easily integrated with selenium.
Sample code from git :
// start the proxy
BrowserMobProxy proxy = new BrowserMobProxyServer();
proxy.start(0);
// get the Selenium proxy object
Proxy seleniumProxy = ClientUtil.createSeleniumProxy(proxy);
// configure it as a desired capability
DesiredCapabilities capabilities = new DesiredCapabilities();
capabilities.setCapability(CapabilityType.PROXY, seleniumProxy);
// start the browser up
WebDriver driver = new FirefoxDriver(capabilities);
proxy.enableHarCaptureTypes(CaptureType.REQUEST_CONTENT, CaptureType.RESPONSE_CONTENT);
// create a new HAR with the label "yahoo.com"
proxy.newHar("yahoo.com");
// open yahoo.com
driver.get("http://yahoo.com");
// get the HAR data
Har har = proxy.getHar();
Hope that will helps.