The following example retrieves the class of the instance that invoked the workForCaller method. Then, its class loader is retrieved and supplied to the Class. Thus, the resolver used for looking up the class is the resolver of the calling class.
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You can resolve the problem of where to find the class by either supplying the resolver, which knows the schemas to search, or by supplying the schema in which the class is loaded. If you know in which schema the class is loaded, you can use the classForNameAndSchema method. Oracle9 i provides a method in the DbmsJava class, which takes in both the name of the class and the schema in which the class resides. This method locates the class within the designated schema. The following example shows how you can save the schema and class names in the save method.
Both names are retrieved, and the class is located using the DbmsJava. You can supply a single String, containing both the schema and class names, to the oracle. When invoked, this method locates the class in the specified schema. The string must be in the following format:. For example, to locate com. You must use uppercase characters for the schema name.
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In this case, the schema name is case-sensitive. When you de-serialize a class, part of the operation is to lookup a class based on a name. In order to ensure that the lookup is successful, the serialized object must contain both the class and schema names. Oracle9 i provides the following classes for serializing and de-serializing objects:.
This class extends java. ObjectOutputStream and adds schema names in the appropriate places. You can use this class in any environment. If used within Oracle9 i , the schema names are read out and used when performing the class lookup. If used on a client, the schema names are ignored. Operating system resources are a limited commodity on any computer. Because Java is targeted at providing a computing platform as well as a programming language, it contains platform-independent classes and frameworks for accessing platform-specific resources.
The Java class methods access operating system resources through the JVM. Java has potential problems with this model, because programmers rely on the garbage collector to manage all resources, when all that the garbage collector manages is Java objects, not the operating system resources that the Java object holds on to.
In addition, because the Oracle9 i JVM is embedded in the database, your operating system resources, which are contained within Java objects, can be invalidated if they are maintained across calls within a session. Oracle9 i manages memory internally, allocating memory as you create new objects and freeing objects as you no longer need them. The language and class libraries do not support a direct means to allocate and free memory.
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Java contains classes that represent file resources. Instances of these classes hold on to your operating system's file constructs, such as file handles, which can become invalid between calls in a session. Java contains classes that represent socket resources. Instances of these classes hold on to socket constructs, some of which can become invalid between calls in a session. See "Sockets" for information specific to maintaining sockets across calls. Threads are discouraged within the Oracle9 i JVM because of scalability issues.
However, you can have a multi-threaded application within the database. By default, a Java user does not have direct access to most operating system resources. A system administrator may give permission to a user to access these resources by modifying the JVM security restrictions.
The JVM security enforced upon system resources conforms to Java 2 security. See "Java 2 Security" for more information. You access operating system resources using the standard core Java classes and methods. Once you access a resource, the time that it remains active usable varies according to the type of resource.
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Memory is garbage collected as described in "Automated Storage Management". Regardless of the usable lifetime of the object for example, the defined lifetime for a thread object , the Java object can be valid for the duration of the session.
This can occur, for example, if the Java object is stored in a static class variable, or a class variable references it directly or indirectly. If you attempt to use one of these Java objects after its usable lifetime is over, Oracle9 i throws an exception. This is true for the following examples:.
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See "Sockets" more information. Imagine that memory is divided into two realms: Java object memory and operating system constructs. The Java object memory realm contains all objects and variables. Operating system constructs include resources that the operating system allocates to the object when it asks. These resources include files, sockets, and so on. Basic programming rules dictate that you close all memory--both Java objects and operating system constructs. Java programmers incorrectly assume that all memory is freed by the garbage collector.
The garbage collector was created to collect all unused Java object memory. However, it does not close any operating system constructs. All operating system constructs must be closed by the program before the Java object is collected.
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For example, whenever an object opens a file, the operating system creates the file and gives the object a file handle. If the file is not closed, the operating system will hold the file handle construct open until the call ends or JVM exits. This can cause you to run out of these constructs earlier than necessary. There are a finite number of handles within each operating system. To guarantee that you do not run out of handles, close your resources before exiting the method.
This includes closing the streams attached to your sockets. You should close the streams attached to the socket before closing the socket. So why not expand the garbage collector to close all operating system constructs? For performance reasons, the garbage collector cannot examine each object to see if it contains a handle. Thus, the garbage collector collects Java objects and variables, but does not issue the appropriate operating system methods for freeing any handles. If you do not close the in file, eventually the File object will be garbage collected.
However, even if the File object is garbage collected, the operating system still believes that the file is in use, because it was not closed. You might want to use Java finalizers to close resources. However, finalizers are not guaranteed to run in a timely manner. Instead, finalizers are put on a queue to execute when the garbage collector has time. If you close your resources within your finalizer, it might not be freed up until the JVM exits.
The best approach is to close your resources within the method. You should close resources that are local to a single call when the call ends.
However, for static objects that hold on to operating system resources, you must be aware of how these resources are affected after the call ends. This can affect any operating system resources within your Java object. For example, if you have a file opened within a static variable, the file handle is closed at the end of the call for you. So, if you hold on to the File object across calls, the next usage of the file handle throws an exception. On the first call, outFile is created. The first input file is opened, read, input into outFile , and the call ends.
Because outFile is statically defined, it is moved into session space between call invocations. However, the file handle--that is, the FileDescriptor --is closed at the end of the call. The next time you call addFile , you will get an exception. There is a workaround: to make sure that your handles stay valid, close your files, buffers, and so on, at the end of every call; reopen the resource at the beginning of the next call.
Another option is to use the database rather than using operating system resources. For example, try to use database tables rather than a file. Or do not store operating system resources within static objects expected to live across calls; use operating system resources only within objects local to the call. The addFile method opens the outme. At the end of each call, the file is closed. Two things occur:. Sockets are used in setting up a connection between a client and a server.
For each database connection, sockets are used at either end of the connection. You might also wish to set up another connection--for example, connecting to a specified URL from within one of the classes stored within the database.