Writing a SimpleDB Client: A Language-Independent Guide in Java

Generator QR Code ISO/IEC18004 in Java Writing a SimpleDB Client: A Language-Independent Guide
10 Writing a SimpleDB Client: A Language-Independent Guide
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Timestamp A string that represents the request time in UTC; for example: 2007-01-31T23:59:59645Z
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In addition to these, there are also request specific parameters that must be set for each operationThese parameters are discussed in detail in 3, A Code-Snippet Tour of the SimpleDB API However, for our purposes here, we need to collect all the parameters and sort them for the signature computation Listing 10-6 shows the start of a Request class to encapsulate the parameter holding, sorting, and signingThis class is declared in the package comsimpledbbookcore In addition to Request, this package will also hold a Response class and an HTTPClient class None of the classes in this package will be seen by users of the client; they are strictly for internal use
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Listing 10-6 Requestjava The Beginning of a SimpleDB Request Class
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package comsimpledbbookcore; import javanet*; import javatext*; import javautil*; import javaxcryptoMac; public class Request { private static final SimpleDateFormat TIMESTAMP; private final SortedMap<String, String> PARAMS; private final Mac HMAC; private final URL URL; static { String format = "yyyy-MM-dd'T'HH%3'A'mm%3'A'ssSSS'Z'"; TIMESTAMP = new SimpleDateFormat(format); TIMESTAMPsetTimeZone(TimeZonegetTimeZone("GMT")); } public Request(Mac hMac, String action, String key, URL url) { URL = url; HMAC = hMac; PARAMS = new TreeMap<String, String>(); PARAMSput("Version", "2009-04-15"); PARAMSput("Timestamp", TIMESTAMPformat(new Date())); PARAMSput("SignatureVersion", "2"); PARAMSput("SignatureMethod", HMACgetAlgorithm()); PARAMSput("Action", action); PARAMSput("AWSAccessKeyId", key); }
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Implementing the Client Code
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public URL getURL() { return URL; } }
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A timestamp format is held in a class constant of type SimpleDateFormat; it can be reused for every request instanceThe format is instantiated and initialized in the static blockThe format string uses the special format defined by the SimpleDateFormat classA notable modification to the format is that the : characters have been replaced with the final URL-encoded value %3A, making encoding unnecessaryAlso, the GMT time zone is set, so the result will be compatible with UTC time In addition to the formatter, there are three final members, one each for Mac and URL passed into the constructor, plus a SortedMap to hold the parametersThe SortedMap is able to store all the parameter names and values in sorted order so that no explicit sorting needs to be done The constructor saves off the URL and the Mac, allocates a concrete instance of SortedMap, and proceeds to insert six parameters Of the seven required parameters, these six are all known in advance, so they might as well be set immediatelyThe version and signature version are constant valuesThe timestamp is created by passing the current time to the formatterThe signature hashing method could conceivably change, so it is pulled directly from the MacThe action and key are variable, but passed to the constructor The two remaining required parameters are the signature and the timestampThe signature cannot be added until the request is complete, but the timestamp could be added now The only concern is that the timestamp defines a window of validity for the request of 15 minutes Even though it is unlikely that 15 minutes would elapse between request creation and transmission, sometimes the unlikely events occur, so the timestamp is set later Setting Optional Parameters Because this Request class needs to handle all SimpleDB operations, it needs a way to accept all of the possible parameters A simple way to do that is to create a method that can accept any set of parameters and add them to the sorted mapThe method could look like this:
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public void setParam(String name, String value) { PARAMSput(name, value); }
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Then the SimpleDB class can set parameters with the following code:
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requestsetParam("DomainName", "users");
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That approach will work, but when you start to think about it, you realize that seven of nine will need to set this domain name parameterThe inconvenience of having to repeat the hard-coded constant DomainName seven times suggests refactoring it into a class constant However, it seems much more appropriate to make the Request class responsible
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