11、JServer JAVA Virtual Machine
JVM是Java技术的基石，它负责实现Java的硬件和操作系统的独立性，保护用户机器不被恶意代码侵害，同时使Java的编译代码足够小。
编译出来的代码足够小是为了方便在网上传播，其实主要是针对Java Applet的，也就是当你需要在客户端下载Java编译后的代码然后运行的时候就
不得不考虑需要下载的文件的大小了。在Java企业级B/S模式下这个问题不是那么突出。
JVM是在不同的硬件和操作系统之上的一个抽象层。它如同OS一样定义了指令集，并管理自己的内存区。Java就运行在这个抽象层之上，
JVM对Java屏蔽了底层硬件和操作系统的不同，这就实现了平台的独立性。同时因为Java运行在JVM上，JVM就像一个沙箱一样控制着Java的行为能力，
也就可以避免恶意的代码侵害底层的OS。其实就是JVM的安全性对Java的行为能力做了限制。
Description: JVM and Jserver
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JVM is a platform-independent execution environment that converts Java bytecode into machine language and executes it.
Most programming languages compile source code directly into machine code that is designed to run on a specific microprocessor architecture or
operating system, such as Windows or UNIX. A JVM -- a machine within a machine -- mimics a real Java processor, enabling Java bytecode to be
executed as actions or operating system calls on any processor regardless of the operating system. For example, establishing a socket connection
from a workstation to a remote machine involves an operating system call. Since different operating systems handle sockets in different ways,
the JVM translates the programming code so that the two machines that may be on different platforms are able to connect.
Oracle JServer Option is a Java Virtual Machine (Java VM) which runs within the Oracle database server's address space.
Oracle also provides a JServer Accelerator to compile Java code natively. This speeds up the execution of Java code by eliminating interpreter overhead.
What is NCOMP:
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The Java language was designed for a platform-independent, secure development model. Translating Java bytecodes into machine instructions degrades performance.
To regain some of the performance loss, you may choose to natively compile certain classes. For example, you may decide to natively compile code with CPU
intensive classes and this can be achived using NCOMP. In 10g one can install NCOMP from companion CD if they are really using JAVA intensive
application and facing any performance issues because of the reason given above.
Note.134985.1 How to use JServer Accelerator and the NCOMP Utility
Schema:
--------
SYS
How to Install and De-Install:
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Note.153271.1 How to manually install JVM into 8i database
Note.126029.1 How to Remove the Jserver (JVM) in versions 8.1.5 and 8.1.6 of the rdbms
Note.175472.1 How to Reload the JVM in 8.1.7.X
Note.202914.1 How to Reload the JVM in 9.0.1.X
Note.209870.1 How to Reload the JVM in 9.2.0.X
Note.276554.1 How to Reload the JVM in 10.1.0.X and 10.2.0.X
Note.457279.1 How to Reload the JVM in 11.1.0.x
Note.1112983.1 How to Reload the JVM in 11.2.0.x
FAQ:
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Note.156439.1 How Does Oracle Store Java Casses Into Database
Note.265293.1 How To Find The Java Source Of A Java Stored Procedure
Note.397770.1 How to Correctly Check if the Oracle JVM is Installed in the Database
Note.471867.1 Simple Java Stored Procedure to Check that the Oracle JVM is Operating Correctly
Note.1098505.1 Master Note for Oracle JVM
Documentation:
----------------
Overview of the Java Virtual Machine (JVM)
12、Oracle XDK
Oracle XDK 组件是一个多功能集合的组件，其可以构建和部署C，C++，Java 程序来处理XML。Oracle XDK 为XML的解决方案提供了基础。
XDK 支持XML DB，XDB是一个技术的集合，其可以用来在DB中存储和处理XML。 我们可以使用XDK 和XDB来构建应用在DB中运行，也可以单独使用XDK。
Description:
-----------
Oracle Oracle XML Developer's Kit (XDK) is a versatile set of components that enables you to build and deploy C, C++, and Java software
programs that process XML. You can assemble these components into an XML application that serves your business needs.
Oracle XDK provides the foundation for the Oracle XML solution. The XDK supports Oracle XML DB, which is a set of technologies used for
storage and processing of XML in the database. You can use the XDK in conjunction with Oracle XML DB to build applications that run in
Oracle Database. You can also use the XDK independently of XML DB.
The Oracle XDK is fully supported by Oracle Corporation and comes with a commercial redistribution license.
The standard installation of Oracle Database includes the XDK.
Note:
Customers using Oracle XDK with PL/SQL and migrating from Oracle Database Release 8.1 or 9.2 are strongly encouraged to use AL32UTF8 as
the database character set. Otherwise, issues can arise during PL/SQL processing of XML data that contains escaped entities.
9206 XDK should be installed in 9204 database, 9207 XDK should be installed in 9205 database and 9208 XDK should be installed in 9206 server.
That is why in 9.2.0.8 installation you will see XDK version 9.2.0.10
Schema:
---------
SYS: In 9.2 XDK should be installed in SYS schema, as 9.2.0.* patchsets upgrade XDK installed in SYS schema only.
How to Install and De-Install:
------------------------------
There is no need to install XDK Java in 10g database as it comes with XDB schema. In 10g, much about XDK has been included in Oracle XML Database (XDB).
Note.317176.1 How to De-install corrupted pl/sql XDK in 9.2 and Install the right version
Note.435999.1 How To Install XDK 10.2.0.x ?
Note.277152.1 How to install XDK 9.2.0.x in an 8.1.7 Database Cleanly
Note.453050.1 How to Verify the XDK Installation
Installing the XDK (11gR2) :
Note.437266.1 Reloading the XDK for Java In the Database
FAQ:
------
Note 250288.1 XDK For PL/SQL API Has Been Deprecated from 9.2 Database
Note.103871.1 XML Parser for PL/SQL FAQs
Note.103872.1 XML Parser for Java Version 2 - Frequently Asked Questions
Documentation:
----------------
Oracle XML DB Developer's Guide, 10g Release 2 (10.2)
13、RAC(Oracle Real Application Clusters)
这是我们最熟悉的组件了，也是安装集群时一定会安装的组件
Oracle Real Application Clusters 实现了标准的、低成本、模块化服务器（如刀片服务器）集群的高利用率。RAC 为服务提供了自动负载管理。服务是一组或一类应用程序，
它们由与应用程序负载相对应的业务组件组成。RAC 中的服务实现了连续、不中断的数据库操作，并支持多个实例中的多个服务。
将服务分配在一个或多个实例以及可以用作备份实例的替代实例上运行。如果主实例出现故障，Oracle 将把服务从故障实例转移到仍旧运行的替代实例上。
Oracle 还自动在运行着服务的实例间进行连接负载平衡。
RAC 利用多台低成本计算机一同工作，作为一台大型计算机来进行数据库处理，这是大型 SMP 系统唯一可行的替换方案，可用于各种应用程序。
基于共享磁盘架构的 RAC 可以根据需要扩大和缩小，而不必在集群服务器之间采用人工方式对数据进行分区。RAC 提供了一键式向集群中添加服务器和从中删除服务器的功能。
因此，可以轻松地向数据库提供服务器或从中删除服务器。
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