驱动程序类如何位于JDBC4中-HowisdriverclasslocatedinJDBC4

10  

Some information about JDBC4 driver loading taken from : http://www.onjava.com/2006/08/02/jjdbc-4-enhancements-in-java-se-6.html

关于JDBC4驱动程序加载的一些信息来自:http://www.onjava.com/2006/08/02/jdbc -4-enhancements in java-se-6.html

When the method getConnection is called, the DriverManager will attempt to locate a suitable driver from among the JDBC drivers that were loaded at initialization and those loaded explicitly using the same class loader as the current application.

当调用getConnection方法时，驱动程序管理器将尝试从初始化时加载的JDBC驱动程序和使用与当前应用程序相同的类加载程序显式加载的JDBC驱动程序中找到合适的驱动程序。

The DriverManager methods getConnection and getDrivers have been enhanced to support the Java SE Service Provider mechanism (SPM). According to SPM, a service is defined as a well-known set of interfaces and abstract classes, and a service provider is a specific implementation of a service. It also specifies that the service provider configuration files are stored in the META-INF/services directory. JDBC 4.0 drivers must include the file META-INF/services/java.sql.Driver. This file contains the name of the JDBC driver's implementation of java.sql.Driver. For example, to load the JDBC driver to connect to a Apache Derby database, the META-INF/services/java.sql.Driver file would contain the following entry:

驱动管理器方法getConnection和getDrivers被增强以支持Java SE服务提供者机制(SPM)。根据SPM，服务定义为一组众所周知的接口和抽象类，服务提供者是服务的特定实现。它还指定服务提供者配置文件存储在META-INF/services目录中。JDBC 4.0驱动程序必须包含文件META-INF/services/java.sql.Driver。这个文件包含JDBC驱动程序实现java.sql.Driver的名称。例如，要加载JDBC驱动程序以连接到Apache Derby数据库，即META-INF/services/java.sql。驱动程序文件将包含以下条目:

org.apache.derby.jdbc.EmbeddedDriver

Now coming to your question.

现在来回答你的问题。

My question is how? What if there are multiple drivers in the classpath?

我的问题是如何?如果类路径中有多个驱动程序怎么办?

As a class loader rule, any class found first will be loaded and if it is already loaded then will not be reloaded by the class loader.

作为类装入器规则，首先找到的任何类都将被装入，如果它已经装入，那么类装入器将不会重新装入。

10  

Every JDBC 4 compliant driver has a file in its jar called META-INF/services/java.sql.Driver, in that file it will list its implementation(s) of java.sql.Driver. When you request a connection, DriverManager will use the ServiceLoader to find all(!) copies of META-INF/services/java.sql.Driver in the classpath and will then load all classes listed. When a java.sql.Driver class is loaded, it has to register itself with the DriverManager, so the DriverManager loads all classes using the service loader, and each Driver implementation registers itself.

每个兼容JDBC 4的驱动程序在其jar中都有一个名为META-INF/services/java.sql的文件。在该文件中，它将列出java.sql.Driver的实现。当您请求一个连接时，DriverManager将使用ServiceLoader来找到META-INF/services/java.sql的所有(!)副本。驱动程序在类路径中，然后将加载列出的所有类。当一个java.sql。驱动程序类被加载，它必须向驱动程序管理器注册自己，因此驱动程序管理器使用服务加载器加载所有类，每个驱动程序实现注册自己。

When you request a connection from DriverManager, the DriverManager will iterate over all registered drivers asking them for a Connection. The driver will use the JDBC url to check if its a protocol it supports (eg Jaybird/Firebird JDBC checks if the url starts with "jdbc:firebirdsql:" or "jdbc:firebird:"). If the driver does not support the protocol, it will return null, if it does support the protocol it will either return an established connection, or it will throw an SQLException (eg if you made an error in the URL, or it couldn't connect). If all drivers return null (none support the protocol), then DriverManager will throw an SQLException with error "No suitable driver found for "

当您从驱动管理器请求连接时，驱动管理器将遍历所有请求连接的已注册驱动程序。驱动程序将使用JDBC url检查它是否支持它的协议(例如Jaybird/Firebird JDBC检查url是否以“JDBC:firebirdsql:”或“JDBC: Firebird:”开头)。如果驱动程序不支持协议，它将返回null，如果它支持协议，它将返回一个已建立的连接，或者它将抛出一个SQLException(例如，如果URL出错，或者它无法连接)。如果所有的驱动程序都返回null(不支持该协议)，那么DriverManager将会抛出一个带有错误的SQLException，“不适合在 中找到合适的驱动程序”。

So having multiple drivers on the classpath does not matter as long as they support different protocols, however if there are multiple drivers for the same database (or at least: same protocol prefixes), it will use the first in the list of drivers, and if that driver fails with an SQLException, it will not try the other one.

所以有多个驱动程序类路径无关紧要,只要他们支持不同协议,但是如果有多个驱动程序相同的数据库(或者至少:协议前缀相同),它将使用列表中的第一个司机,如果司机没有抛出SQLException,它不会尝试另一个。

0  

In JDBC 4, the drivers are registered automatically. How this happens? When we write

在JDBC 4中，驱动程序是自动注册的。这是如何发生的?当我们写

Connection con=DriverManager.getConnection("jdbc:mysql://localhost:3306/sonoo","root","root");  

this will call the static blocks inside the DriverManager class. If you open the DriverManager source file, you can see the static block

这将调用DriverManager类中的静态块。如果打开DriverManager源文件，您可以看到静态块

 static {
        loadInitialDrivers();
        println("JDBC DriverManager initialized");
    }

this will call the function loadInitialDrivers(); function and load all the drivers and registered automatically. So we don't need to explicitly register or load the drivers.

这将调用函数loadInitialDrivers();运行并加载所有驱动程序并自动注册。所以我们不需要显式地注册或加载驱动程序。