visual basic 6.0 barcode generator 4: Creating a Download Manager in Java in Java

Printing PDF 417 in Java 4: Creating a Download Manager in Java

4: Creating a Download Manager in Java
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The run( ) Method
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When the run( ) method executes, the actual downloading gets under way. Because of its size and importance, we will examine it closely, line by line. The run( ) method begins with these lines:
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RandomAccessFile file = null; InputStream stream = null; try { // Open connection to URL. HttpURLConnection connection = (HttpURLConnection) url.openConnection();
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First, run( ) sets up variables for the network stream that the download s contents will be read from and sets up the file that the download s contents will be written to. Next, a connection to the download s URL is opened by calling url.openConnection( ). Since we know that the Download Manager supports only HTTP downloads, the connection is cast to the HttpURLConnection type. Casting the connection as a HttpURLConnection allows us to take advantage of HTTP-specific connection features such as the getResponseCode( ) method. Note that calling url.openConnection( ) does not actually create a connection to the URL s server, it simply creates a new URLConnection instance associated with the URL that later will be used to connect to the server. After the HttpURLConnection has been created, the connection request property is set by calling connection.setRequestProperty( ), as shown here:
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// Specify what portion of file to download. connection.setRequestProperty("Range", "bytes=" + downloaded + "-");
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Setting request properties allows extra request information to be sent to the server the download will be coming from. In this case, the Range property is set. This is critically important, as the Range property specifies the range of bytes that are being requested for download from the server. Normally, all of a file s bytes are downloaded at once. However, if a download has been interrupted or paused, only the download s remaining bytes should be retrieved. Setting the Range property is the foundation for the Download Manager s operation. The Range property is specified in this form: start-byte end-byte For example, 0 12345 . However, the end byte of the range is optional. If the end byte is absent, the range ends at the end of the file. The run( ) method never specifies the end byte because downloads must run until the entire range is downloaded, unless paused or interrupted. The next few lines are shown here:
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// Connect to server. connection.connect();
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// Make sure response code is in the 200 range. if (connection.getResponseCode() / 100 != 2) { error(); } // Check for valid content length. int contentLength = connection.getContentLength(); if (contentLength < 1) { error(); }
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The connection.connect( ) method is called to make the actual connection to the download s server. Next, the response code returned by the server is checked. The HTTP protocol has a list of response codes that indicate a server s response to a request. HTTP response codes are organized into numeric ranges of 100, and the 200 range indicates success. The server s response code is validated for being in the 200 range by calling connection.getResponseCode( ) and dividing by 100. If the value of this division is 2, then the connection was successful. Next, run( ) gets the content length by calling connection.getContentLength( ). The content length represents the number of bytes in the requested file. If the content length is less than 1, the error( ) method is called. The error( ) method updates the download s status to ERROR, and then calls stateChanged( ). The stateChanged( ) method will be described in detail later. After getting the content length, the following code checks to see if it has already been assigned to the size variable:
/* Set the size for this download if it hasn't been already set. */ if (size == -1) { size = contentLength; stateChanged(); }
As you can see, instead of assigning the content length to the size variable unconditionally, it only gets assigned if it hasn t already been given a value. The reason for this is because the content length reflects how many bytes the server will be sending. If anything other than a 0-based start range is specified, the content length will only represent a portion of the file s size. The size variable has to be set to the complete size of the download s file. The next few lines of code shown here create a new RandomAccessFile using the filename portion of the download s URL that is retrieved with a call to the getFileName( ) method:
// Open file and seek to the end of it. file = new RandomAccessFile(getFileName(url), "rw"); file.seek(downloaded);
The RandomAccessFile is opened in rw mode, which specifies that the file can be written to and read from. Once the file is open, run( ) seeks to the end of the file by calling
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