Using External Data Sources in Java

Making Quick Response Code in Java Using External Data Sources
27 Using External Data Sources
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Table 27-1 Variables and Callback Functions That Track the Status of an HTTP GET Request (Continued) Variable Variable Type Function Function Argument Type
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readingHeaders Boolean onReadingHeaders
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Set/Function Called
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None
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When the HTTP headers sent by the server are being read The headers that are defined by the HTTP 11 protocol specification are all represented by constants declared by the HttpHeaders class When the server s response code has been received The possible values are defined as constants in the HttpStatus class When the server s response message has been received The response message is a textual representation of the response code The value of an error response from the server, if there is one When the response headers are available to be read When all the HTTP headers have been processed When the body of the response is being read When the total number of bytes of response data is known
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responseCode
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Integer
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onResponseCode
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responseMessage
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String
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onResponseMessage
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String
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error
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Input Stream
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onError
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Input Stream String[]
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onResponseHeaders onDoneHeaders
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doneHeaders
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Boolean
None
reading
Boolean
onReading
None
toRead
Long
onToRead
Long
The HttpRequest Class
Table 2 7e Var iab le s and Callba ck Fu nctio ns That Track the Status of an a a e d b k u o t HTTP G E R eques (Continued) PT e Variable Variable Type Function Function Argument Type
read Boolean onRead Long
Set/Function Called
As bytes of data are received from the server When the response body is available to be read This variable and its associated callback function are used only when the sink variable is not set See the section A GET Request to a Valid HTTP URL for details When all the body has been read When the operation is complete, successfully or otherwise
input
InputStream
onInput
InputStream
doneRead done
Boolean Boolean
onDoneRead onDone
None None
On completion of the operation, whether or not it is successful, the done variable is set to true and the onDone function is calledThis is always the final step in the lifecycle It is important to note that, like all JavaFX objects, an HttpRequest must be created and used exclusively in the main thread of the JavaFX applicationThe process of connecting to the web server and reading/writing data are actually performed in a background thread, but the variables and callback functions in the HttpRequest object are always set and called in the main thread of the applicationThis means that application code and triggers are always called in the main thread, which is necessary because JavaFX does not support access to data from more than one thread If you refer back to Listing 27-1, the process of reading the image data is initiated by the invocation of the start() function on line 25, but it will not be complete when this function returns receipt of data will be notified to the onDoneRead function at some future point and completion of the operation to the onDone function, which is not used in this example
GET Requests
The code in the file javafxdata/HttpRequestExample2fx creates a user interface with an input field into which you can type a URL, a button that can be pressed to perform a
27 Using External Data Sources
request to that URL, and a textbox in which the results of the operation are displayedThe doRequest() function builds the HttpRequest, attaches triggers to the variables, and installs callbacks in all the function variables listed in Table 27-1The triggers and callback functions all write tracking information to the textbox so that the order in which these events occur can be seen Here is an extract from this function that shows a typical callback function and trigger:
GET function doRequest(url:String) { r = HttpRequest { location: url sink: new ByteArrayOutputStream() // Callback functions onConnecting: function() { addText("onConnecting\n"); } // // Code omitted // // State variables var connecting = bind rconnecting on replace { addText("connecting -> {connecting}\n") // // Code omitted // } rstart(); }
When a callback function is invoked, its name and relevant information derived from its argument, if it has one, are recorded in the textboxWhen the value of a state variable changes, its name and value are shown, separated by an arrow In the sections that follow, we use this code to examine the results of several different GET requests Figure 27-2 shows some typical output A GET Request to an Invalid URL For our first example, we enter the URL http://nosuchurl into the text field and click the Go buttonThis causes the following to be written to the textbox5: