Using External Data Sources in Java

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That access to JavaFX variables is made only from the application s main thread If this were not the case, there would either need to be language-level support for synchronization, or the compiler and runtime would have to cooperate somehow to transparently guarantee thread safety (which they do not) That JavaFX code is executed only in the application s main thread
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The consequence of the first assumption is that the variable updates required in step 5 cannot be made directly from the background thread, and the result of the second assumption is that the code that runs in the background thread has to be written in Java instead of JavaFX Writing the code that runs in the background thread in Java causes another problem, arising from the fact that in step 3, we have to create a JavaFXCustomer object for each row of the CUSTOMERS tableThe problem is that creating a JavaFX object in a background thread is not allowedWe can solve this problem by creating a Java class that is equivalent to the JavaFXCustomer class, which will have the same variables and which will be called JavaCustomer to make clear that it is a Java classAs you ll see later, the JavaCustomer objects will be converted to JavaFXCustomer objects at step 5 of the process It follows that we need at least four classes to implement the FetchCustomersTask:
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The FetchCustomersTask class itself, which is a JavaFX class The JavaFXCustomer class, which is another JavaFX class The class that contains the code that will run in the background thread, which must be a Java class The JavaCustomer class, which is also a Java class
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In fact, as you ll shortly see, we also need to create a Java interface that will allow Java code to update the state of the FetchCustomersTask object
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Implementing the Database Access Task
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The JavaTaskBase class in the javafxasync package is intended to be used as the base class for all asynchronous operations It derives from Task and adds one additional function, which must be implemented by subclasses:
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protected abstract function create():RunnableFuture;
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This function is required to return the object that contains the code that will be run in the background threadAs we have already seen, this class must be written in Java, and the signature of the create() function requires it to implement the javafxasyncRunnableFuture interface, which consists of a single method:
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public abstract void run() throws Exception;
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If this function returns a JavaFX object, an exception will be thrown It is, therefore, not possible to implement the background task in JavaFX
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Asynchronous Operations and Database Access
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The JavaTaskBase class arranges for the create() function to be called and the code in the run() method of the returned Java object to be executed in a background thread If this method completes normally, it is assumed that the operation succeeded and the succeeded and done variables of the task will be set to true If it throws an exception, that exception will be stored in the causeOfFailure variable of the task, and the failed and done variables will be set to true The implementation of the FetchCustomersTask class is shown in Listing 27-15
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Listing 27-15
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The FetchCustomersTask Class
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public class FetchCustomersTask extends JavaTaskBase, TaskUpdater public-init var dbURL:String; public-init var params:Pair[]; public-read var customers:JavaFXCustomer[]; public var onCustomers:function(JavaFXCustomer[]):Void; protected override function create():RunnableFuture { var props = new Properties(); for (param in params) { propsput(paramname, paramvalue); } new JavaCustomerFetcher(dbURL, props, this); } // Implementation of TaskUpdater interface public override function setReturnValue(value:Object) { var javaCustomers = value as nativearray of JavaCustomer; for (javaCustomer in javaCustomers) { insert JavaFXCustomer { customerId: javaCustomercustomerId; name: javaCustomername; } into customers; } if (onCustomers != null) { onCustomers(customers); } } public override function setProgress(progress:Long, maxProgress:Long) { thisprogress = progress; thismaxProgress = maxProgress; } }
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