How does jdbc work_问答_开发者_运维开发者技术经验分享

can anyone tell me How does jdbc work? How it manages to communicate with a DBMS? since DBMS may be writte开发者_开发知识库n with some other programming language.

Communication with the database is handled by JDBC drivers that can use various strategies to "talk" to a database (from "translation" to the use of "native" language). Depending on the strategy used, drivers are categorized into 4 types. Types of JDBC technology drivers provide a good description of each of them:

A JDBC-ODBC bridge provides JDBC API access via one or more ODBC drivers. Note that some ODBC native code and in many cases native database client code must be loaded on each client machine that uses this type of driver. Hence, this kind of driver is generally most appropriate when automatic installation and downloading of a Java technology application is not important. For information on the JDBC-ODBC bridge driver provided by Sun, see JDBC-ODBC Bridge Driver.

A native-API partly Java technology-enabled driver converts JDBC calls into calls on the client API for Oracle, Sybase, Informix, DB2, or other DBMS. Note that, like the bridge driver, this style of driver requires that some binary code be loaded on each client machine.

A net-protocol fully Java technology-enabled driver translates JDBC API calls into a DBMS-independent net protocol which is then translated to a DBMS protocol by a server. This net server middleware is able to connect all of its Java technology-based clients to many different databases. The specific protocol used depends on the vendor. In general, this is the most flexible JDBC API alternative. It is likely that all vendors of this solution will provide products suitable for Intranet use. In order for these products to also support Internet access they must handle the additional requirements for security, access through firewalls, etc., that the Web imposes. Several vendors are adding JDBC technology-based drivers to their existing database middleware products.

A native-protocol fully Java technology-enabled driver converts JDBC technology calls into the network protocol used by DBMSs directly. This allows a direct call from the client machine to the DBMS server and is a practical solution for Intranet access. Since many of these protocols are proprietary the database vendors themselves will be the primary source for this style of driver. Several database vendors have these in progress.

As we can see, there are various strategies to make interoperability possible, including implementing the network protocol used by a given database in Java (type 4). And because of their ease of use (no extra stuff to install, no JNI) and their good performances (they perform as well as type 2 drivers now), type 4 are actually the most frequently used nowadays.

From Wikipedia:

JDBC drivers are client-side adapters (installed on the client machine, not on the server) that convert requests from Java programs to a protocol that the DBMS can understand. [edit] Types

There are commercial and free drivers available for most relational database servers. These drivers fall into one of the following types:

Type 1 that calls native code of the locally available ODBC driver.

Type 2 that calls database vendor native library on a client side. This code then talks to database over network.

Type 3, the pure-java driver that talks with the server-side middleware that then talks to database

Type 4, the pure-java driver that uses database native protocol

Most database systems support ODBC (Open Database Connectivity or whatever). This is meant to allow applications (e.g., Access) to work with multiple RDBMS implementations at the cost of some performance hit. When JDBC was first released, there was a driver that allowed you to connect to an ODBC provider. Later on, some vendors provided JDBC drivers specific to their RDMS.

From a developer's perspective, JDBC is used as a set of interfaces. All the actual details are hidden in loading the driver. The driver is a Java class that can use any trick of the book, including native code or just sending network traffic to the RDBMS.

From Wikipedia:

JDBC is an API for the Java programming language that defines how a client may access a database. It provides methods for querying and updating data in a database. JDBC is oriented towards relational databases.

JDBC was first introduced in the Java 2 Platform, Standard Edition, version 1.1 (J2SE), together with a reference implementation JDBC-to-ODBC bridge, enabling connections to any ODBC-accessible data source in the JVM host environment.

Without going into too much detail, you can think of JDBC as an abstraction layer that lets you talk to different databases. The implementation-specific details are hidden from you, but the interface for querying a database (be it MySQL or Oracle or whatever) is the same.

What this means is that in future, if there was a new database, someone only needs to use the existing interface. The method names would be the same, but the methods would contain implementation-specific code for that particular database. This is a common software-engineering pattern.

The entity that contains the implementation-specific code is called the JDBC driver. The JDBC driver provides a connection to the database and it also implements the specific protocol for sending the query to the database and the result-set back to the client.

Can't say I know the exact answer to your question, but here is some information to help.

Here's a great place to start:

http://java.sun.com/products/jdbc/overview.html

The JDBC API contains two major sets of interfaces: the first is the JDBC API for application writers, and the second is the lower-level JDBC driver API for driver writers.

Information for JDBC Driver Developers. Basically there are a set of interfaces that a developer implements to create a JDBC driver for a particular DBMS.

http://java.sun.com/products/jdbc/driverdevs.html

As far as a DBMS being written in a different language. That DBMS most likely exposes some API (in various languages and/or formats) that allow for drivers, such as JDBC, to communicate with the DBMS.

From the wikipedia page:

Types

There are commercial and free drivers available for most relational database servers. These drivers fall into one of the following types:
Type 1 that calls native code of the locally available ODBC driver.
Type 2 that calls database vendor native library on a client side. This code then talks to database over network.
Type 3, the pure-java driver that talks with the server-side middleware that then talks to database
Type 4, the pure-java driver that uses database native protocol

Reading a bit about the four types of JDBC drivers might enlighten you.