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[Music] welcome to Carrboro in this tutorial we're going to talk about spring boo trust content negotiation we'll talk about various techniques involved in content negotiation along with some really good examples so here is a high-level overview of the entire content that will be covering as part of this tutorial first of all we'll try to understand what is content negotiation we'll talk about how we can do content negotiation using HTTP headers request parameters and path extensions and then we'll try to understand what is content type header produces attribute we'll also take a look at how we can generate PDF response from this server and then finally we're going to talk about accept encoding for compression algorithm we lunker all these concepts with core examples understanding about content negotiation is very crucial for developers because it is used in day-to-day real-time projects and it's also very crucial for interviews my goal of this tutorial is to make you understand what is content appreciation in Springwood rest api along with core examples and you will learn better if you avoid any distractions like mobile phone notifications Facebook feed or YouTube etc and stay focused on this tutorial till the end it is for your own benefit and after you finish watching I kept some links in the description with some freebies and free courses etc do check them out it may be of helpful to you with that in mind let's get started let's talk about content negotiation in restful web services we already know that in case of REST API the client can be anything it could be a JavaScript application it could be a mobile application or it can even be traditional MVC application in case of traditional MVC application the browser would send the request and it would expect HTML as the response from the server in order for it to render on the browser but in other cases the format can be different for example if you're talking about a JavaScript application it would expect the server to respond back in either XML or in JSON format so it's not specific what kind of format that the client needs in this case it may not be ideal to configure a specific format on the server side instead we want to leave that option to the client to specify what kind of format it is expecting from the server and at the same time server may not be capable of sending in any random format that ever existed so there are certain limitations on the server side as well if the client requests for a format that server doesn't support so we would just simply respond back with an error so this mechanism is called content negotiation so how will the server know what kind of format that the client is requesting for well there are three different mechanisms for that the first of which his so-called path extension who just simply means that the client would specify the format as part of the path extension an example of which can be seen in here in this case the client is requesting lists of products in the format of PDF and so we would be able to determine that by taking a look at this extension hit the server supports PDF format then the server would respond back with PDF as a response similarly the client can request in JSON format in which case server would respond back in the format of JSON another approach is by using a URL parameter an example of which can be seen in here pretty much same as the previous one except we're not going to include the path extension instead we would include a URL parameter with the name format this is customizable by the way and we'll take a look at it in our example so in this case client is requesting for the PDF format and here client is requesting for XML format yet another approach is to include a history we accept header obviously can't use browser in order to include HTTP headers he have to use a tool like postman or JavaScript in order to do so but essentially what you need to do is to include a property as part of the HTTP headers and the name of property is except which tells the server what kind of format client is willing to accept and the value of it is going to be the format that the client is willing to consume once we take a look at an example you'll understand this better let's talk about message converters first of all what is a message a message in web service is simply a request or response having information to process or retrieve the data I mean when you're sending esterday requests from the client that's called a message it's called a request message which will have enough information for the server to process and respond back with something and when server responds back with something as part of the HTTP response it's called a response message but how does this conversion happen I mean you've seen in our crud example that we were able to include JSON data as part of the request and the server is able to convert that JSON data into a Java model object in order for it to process how does that conversion happen well that is with the help of these converters in case of JSON we have a specific converter for that similarly we have a convertor for XML similarly for all the other formats marshaling would just simply mean the process of converting the json or xml or representation to equivalent java object and we've seen this example when they're trying to send the json data from the client and sort of updated a record on the server side and the opposite of it is called an marshalling which is simply the process of converting java object back to the representation I mean when the controller is returning a model object somebody is trying to represent that object in the client requested format like JSON or XML that mechanism is called an marshaling okay let's first explore has to DP had a way of containing ago she ation what I have here is a crud API that we are developed previously the first thing that you need to do is to go to form the XML file and add this dependency by default when you create a springboard project with starter web dependency it comes with JSON support but if you want to be able to represent your data or the resource in XML format then you have to add this additional dependency otherwise things may not work as expected once you add this dependency do make sure that you update the project so that libraries would be present in your class path let's run our application and see how we can configure yesterday P headers and specify the accept property I would again go to postman let me send the request and by default it's going to respond back with JSON data but let's say that I wanted to have this next ml format I go to header stab and I would specify the property except you already have it populated just click it and you would specify the content type in this case that's application and if you notice there are a lot of options in here you can choose one of them but of course your API has to support it as well in my case I'm going to change this to XML and now if I fire the request everything is shown in XML format so what has happened behind the scenes is when I send this request spring is going to take a look at HTTP headers especially this property in here it can then figure out the expected media type that the trend can accept in this case it's application slash XML and since we've added this library as part of our XML file we do have an XML converter that can convert our Java object into bunch of XML tags and using that converter whatever we're trying to respond back in here how rather here would be converted and represented in XML format and that's what we're seeing in here and from this point on for all the requests the content or the response would be represented in XML format let's take a look at how we can make use of request parameter for content negotiation by default this feature is disabled so we have to enable it with our configuration if you're not using springboard and you're developing spring applications that ready no way then you have to introduce quite a bit of configurations either in Java config or in XML config but with spring boot things are going to be a lot easier all you have to do is to add these properties inside application or properties file if you created the project using spring to suit you should be able to locate this file under resources folder so with this property we are trying to enable that feature and with this property we can customize the name of that parameter this is optional if you don't specify this property the default name would be format but in this case the parameter name has to be media type let's take a look at how we can use it let's relaunch our application go to postman this time I'm going to include this as a parameter currently I have has two DP headers as well and I'm trying to accept XML as the format but let's see what's going to happen if I use a parameter and make it JSON well it looks like if you use the parameter then this will have preference over HTTP headers so if I change this to XML response would be represented in XML format regardless of what you configure in here let's talk about path extension way of content negotiation by default that feature is disabled and if you want to enable it you just have to add this property inside your application or properties file I need to set this flag to true from this point on he should be able to do content negotiation with extensions and this will have precedence toward the other two methods that we had talked when I tried to test our application after configuring this I noticed that our application is not working as expected when I checked with one of my associates he conformed me that this is an existing bug in Spring Framework and the team is actively looking at fixing it but I really should be able to append an extension like dot JSON or XML or PDF or whatever and accordingly you should be able to see the response in specified format you can give it a try maybe by the time you're watching this video it may have got fixed but do make sure that you got the latest version of Spring Framework just as the server is able to respond back with data in various different formats or media types when client can send data in various different formats and several must be able to receive and process it one of the cases where this might happen his when client is trying to aid their update or create a resource so let's try to update an existing resource going to go to postman and he wrote the list of products available and/or despair in XML format I went to copy one of these because I want to update this particular record I changed the method to put inside the body I want to change this to something else whatever and if I try to send the request I went to see an error it says the content type not supported the real cause of this error is that server is not able to pick the right converter to convert the XML into Java object so Cline has to specify the type of content it is trying to send by adding one additional header with the name content type and then you specify the type of content you're trying to send now this time it worked I was mentioning before that we can have restrictions on the server side as to what kind of media type that it is capable of handling at the moment or get method is capable of responding back with either JSON or XML but let's say for some reason I want to restrict it to respond back with only XML data or say JSON how do I apply that restriction on this method well it's by using the property producers and your specified list of media types but this method is capable of producing the reason why you're seeing this error is because when you have more than two attributes then you have to provide value for all the attributes that are present in this annotation so this is going to be value so now this method is capable of producing either of these media types so this method essentially is going to exhibit pretty much the same behavior has with our previous examples so let's try to apply restrictions and say that this method is capable of generating only json media type let's save the file and run it let's go to postman and if you send the request it's going to work because JSON is supported let's change the accept header to XML and see what's going to happen you're probably going to see an error and it says could not find acceptable representation with four or six STP status so that explains you can also have this attribute added at class level so that it would be applicable in all the methods residing in this class one of the cases where this might come in handy is when you want to call a specific method depending on the type of media type that it can handle so let me just quickly copy this an obviously signature has to change to something else and let me change this to XML for instance so now we have a couple of methods if the client is requesting for JSON data this method would be triggered or if the client is expecting XML data then the request would end up in here let's relaunch our application and this time we shouldn't have any problem responding back with XML data in this case however the behavior of both these methods is exactly the same and this may not imply a real-time scenario so for this purpose in order to demonstrate I'm going to introduce a method that's going to respond back with a PDF media type I already have the code copied just to save your time and here it is in addition to this I've also created a PDF file and I've put it inside the resources folder under files you can put any content inside this PDF it doesn't matter but what we're trying to do here is when we try to send a get request requesting for a PDF media type then this method will be triggered and ultimately is in streams who are trying to respond back with PDF data now I really hope that you don't expect me to walk you through all this code this is all core Java all we're trying to do is using streams who are trying to read that file and make it part of the response using this response entity class you'll see more examples of response entity incoming times so here is a PDF file we're trying to pick from the class path and then I'm trying to add a header which is basically the size or the content length of the PDF file we'll be able to see this header as part of the response headers and I'm using this response entity object to sort of create the response with headers as well as the status code let's relaunch our application and this time XML would obviously fail let me chain the headers the accept error to PDF and send the request again well if you notice you don't see anything in here because postman client is not capable of displaying the PDF content but you can say that it has worked because of the status code 200 okay along with that you'll also see the header which is a content length of that PDF so that explains let's talk about compression mechanisms real quick in our case our application is running in the same host and even the response is just in few lines however in real time scenario the server might be residing in different geographical location and even the response can be in hundreds of lines in which case it takes a while for the response to be transmitted over the Internet to the client and if there are millions of requests to the server it's going to cost a lot of bandwidth and delay in response in order to address this issue we have compression mechanism in place let me show you what I mean if you go to the network's tab having same tool in Chrome as Ville and I'm guessing that you're aware of it to take a look at the response you would notice a request header that says accept encoding and specifies couple of encoding mechanisms or encoding algorithms that the client is expecting server to use in order to compress the response so using this header server is going to pick one of these algorithms in order to compress the response so that when the response reaches to the client client is going to use the same algorithm in order to decode the response and render back the media in this manner it's all taken care Ematic alee you don't have to add any additional headers or anything of that sort [Music]