Local Storage in Java

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Local Storage
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Most applications require persistent storage If you are writing a game, you probably want to save a list of high scores, or if you are writing a business application, you might want to save the user s preferencesAccess to local storage on the user s device is problematic for a couple of reasons:
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The storage mechanism is platform-dependent and so are the APIs used to access it On a desktop system, persistent data is stored in files, but on a mobile device, nonvolatile memory is used instead Access to local storage is usually restricted, for security reasons For example, unsigned applications and applets are not allowed access to the user s file system, to prevent unauthorized reading or modification of data
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The JavaFX runtime provides a simple local storage facility that addresses both of these issuesThe public API consists of only two classes, which delegate to platform-dependent implementations that work with whatever storage mechanism exists on the host device, thus addressing the issue of platform-independence Security concerns are addressed by not allowing the application free access to whatever local storage is present Instead, data is stored in an area of the file system or nonvolatile memory that is entirely managed by the JavaFX runtime It is impossible for an untrusted JavaFX application to read or write data outside of this dedicated area, so there is no possibility that it could violate the security of
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Local Storage
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the user s device Because the local storage mechanism is completely under the control of the JavaFX runtime, you do not need to sign your application to make use of it
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Reading and Writing Data
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To write some data to local storage, you first need to create an instance of the javafxioStorage class, giving the name of the resource5 to which you want to write As you ll see in the section Resource Names and Access Control, later in this chapter, the resource name can either be relative or absolute For simplicity, we ll confine ourselves to the use of relative resource names for now Once you have a Storage object, you can get a Resource object that lets you read and write the content of the underlying storage Here s a simple example that illustrates the API6:
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 package platformapi; import javafxioStorage; import javaioDataOutputStream; import javalangSystem; var s = Storage { source: "Hello"; } var r = sresource; var os = new DataOutputStream(ropenOutputStream(true)); oswriteLong(SystemcurrentTimeMillis()); oswriteUTF("Hello, World"); osclose(); println("Entry written");
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The Storage object is created on lines 7 to 9The source variable is initialized with the name of the resource, which in this case is called "Hello"A relative resource name must not contain a / character because these are used to separate the elements of an absolute name, examples of which you will see later in this chapter During initialization, the Storage object creates a javafxioResource object that refers to the storage resource named by the source variable and stores it in its resource variable One lines 11 and 12, we get the Resource instance and use its openOutputStream() function to get an OutputStream that can be used to write to the resource:
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public function openOutputStream(overwrite:boolean):OutputStream;
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Throughout this section, we use the term resource rather than file to emphasize the fact that the actual representation of the resource may not be a file 6 You ll find this example in the file LocalStorage1fx in the platformapi package of the JavaFX Book Language project
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12 Platform APIs
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If the resource does not yet exist, it is created automatically If it does exist, its content may or may not be overwritten, depending on the value of the overwrite argument If this argument is true, any existing content will be lost; if it is false, the existing content will be preserved and new data is appended to it Any data written to the OutputStream is stored in the resource and flushed when the OutputStream is closed Writing directly to an OutputStream is not very convenient because its methods deal only with bytes and byte arraysThere are various ways to write a String to an OutputStream, one of which is to wrap the OutputStream in an OutputStreamWriter, like this:
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var r = sresource; var w = new OutputStreamWriter(os, "UTF-8"); wwrite("Hello again, world"); wclose();
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Although this approach looks convenient, it works only for StringsTo write a numeric value, you would need to convert it to an array of bytes in some way and then write the array to the OutputStreamYou would also need to implement the reverse conversion to read the value back Such a conversion is already implemented in the DataOutputStream and DataInputStream classes, along with a pair of methods readUTF() and writeUTF() that allow you to read and write StringsThese classes are easy to use and are available on both desktop and mobile devices In the previous example, we use them to write a Long, representing the current time, and a String to the resource:
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var r = sresource; var os = new DataOutputStream(ropenOutputStream(true)); oswriteLong(SystemcurrentTimeMillis()); oswriteUTF("Hello, World"); osclose();
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