JGROUPS源码分析之一：协议栈的创建（一）

先从下面这两行代码为进入点：

1.      JChannel channel = new JChannel();

2.         channel.connect("x");

 

第一行代码创建一个数据传输通道channel，第二行代码表示channel连接到一个名称为x的集群上。

先看 JChannel channel = new JChannel() 做了哪些动作。

 

1.    调用默认的构造函数

public JChannel() throws ChannelException {

        this(DEFAULT_PROTOCOL_STACK);

    }

说明：

DEFAULT_PROTOCOL_STACK为协议栈配置参数，是这样定义的：

public static final String DEFAULT_PROTOCOL_STACK="udp.xml";

 

2.    接着调用一个带有字符串参数的构造函数

public JChannel(String properties) throws ChannelException {

        this(ConfiguratorFactory.getStackConfigurator(properties));

    }

说明：properties为协议栈配置文件，默认为udp.xml

       ConfiguratorFactory是一个协议栈配置工厂类，负责返回一个协议栈配置对象，协议栈对象解析协议，并负责创建协议对象。

3.    看ConfiguratorFactory.getStackConfigurator(properties)调用过程，

/**

     * Returns a protocol stack configurator based on the provided properties

     * string.

     *

     * @param properties an old style property string, a string representing a

     *                   system resource containing a JGroups XML configuration,

     *                   a string representing a URL pointing to a JGroups XML

     *                   XML configuration, or a string representing a file name

     *                   that contains a JGroups XML configuration.

     */

    public static ProtocolStackConfigurator getStackConfigurator(String properties) throws ChannelException {

        if (propertiesOverride != null) {

            properties = propertiesOverride;

        }

 

        // added by bela: for null String props we use the default properties

        if(properties == null)

            properties=JChannel.DEFAULT_PROTOCOL_STACK;

 

        checkForNullConfiguration(properties);

 

        ProtocolStackConfigurator returnValue;

 

        // Attempt to treat the properties string as a pointer to an XML

        // configuration.

        XmlConfigurator configurator = null;

 

        try {

            configurator=getXmlConfigurator(properties);

        }

        catch (IOException ioe) {

            throw createChannelConfigurationException(ioe);

        }

 

        // Did the properties string point to a JGroups XML configuration?

        if (configurator != null) {

            returnValue=configurator;

        }

        else {

            // Attempt to process the properties string as the old style

            // property string.

            returnValue=new PlainConfigurator(properties);

        }

 

        return returnValue;

    }

从这段代码可以看出，先检查协议配置参数，通过则返回一个XML协议栈配置对象configurator。协议栈定义为一个接口

public interface ProtocolStackConfigurator

{

    String         getProtocolStackString();

    ProtocolData[] getProtocolStack(); // ProtocolData对象包含协议名称，实现类等

}

下面重点瞧configurator=getXmlConfigurator(properties);

调用过程，负责解析XML文件：

/**

     * Returns an XmlConfigurator based on the provided properties string (if

     * possible).

     *

     * @param properties a string representing a system resource containing a

     *                   JGroups XML configuration, a string representing a URL

     *                   pointing to a JGroups ML configuration, or a string

     *                   representing a file name that contains a JGroups XML

     *                   configuration.

     *

     * @return an XmlConfigurator instance based on the provided properties

     *         string; <code>null</code> if the provided properties string does

     *         not point to an XML configuration.

     *

     * @throws IOException  if the provided properties string appears to be a

     *