JDK中的设计模式--桥接模式

梦里梦外; 2022-03-17 15:48 253阅读 0赞

java.util.logging是JDK自带的日志包，可以将日志输出到文件、内存或者控制台，作用与我们常用的log4j类似。  
包中的Handler类和Formatter类在设计上利用了桥接模式，首先看类关系图：  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L21pYW95aWJvMTI_size_16_color_FFFFFF_t_70]  
Handle和Formatter类是两个抽象类，它们可以分别独立的变化（有不同的子类）；而Handle类中包含对Formatter类的引用。

//handle类中的set方法
      public synchronized void setFormatter(Formatter newFormatter) throws SecurityException {
            checkPermission();
            // Check for a null pointer:
            newFormatter.getClass();
            formatter = newFormatter;
        }

Handle通过setFormatter方法可以方便的替换不同的Formatter类。

*Handle与Formatter介绍*  
1、Handle.  
来自JDK本身的介绍:

/**
     * A <tt>Handler</tt> object takes log messages from a <tt>Logger</tt> and
     * exports them.  It might for example, write them to a console
     * or write them to a file, or send them to a network logging service,
     * or forward them to an OS log, or whatever.
     * <p>
     * A <tt>Handler</tt> can be disabled by doing a <tt>setLevel(Level.OFF)</tt>
     * and can  be re-enabled by doing a <tt>setLevel</tt> with an appropriate level.
     * <p>
     * <tt>Handler</tt> classes typically use <tt>LogManager</tt> properties to set
     * default values for the <tt>Handler</tt>'s <tt>Filter</tt>, <tt>Formatter</tt>,
     * and <tt>Level</tt>.  See the specific documentation for each concrete
     * <tt>Handler</tt> class.
     *
     *
     * @since 1.4
     */

handle对象可以从一个logger中取出信息并输出到控制台、文件或者调用其它api发送到网络中；  
handle可以通过setLevel方法关闭或打开；  
handle通常是利用LogManager去设置自身的Handler、Filter、Formatter等属性值。  
\**备注*：*虽然父类Handle中提供了setFormatter方法，但子类streamhandle中也重新提供了配置各个成员变量值的方法：*

private void configure() {
            LogManager manager = LogManager.getLogManager();
            String cname = getClass().getName();
            setLevel(manager.getLevelProperty(cname +".level", Level.INFO));
            setFilter(manager.getFilterProperty(cname +".filter", null));
            setFormatter(manager.getFormatterProperty(cname +".formatter", new SimpleFormatter()));
            try {
                setEncoding(manager.getStringProperty(cname +".encoding", null));
            } catch (Exception ex) {
                try {
                    setEncoding(null);
                } catch (Exception ex2) {
                    // doing a setEncoding with null should always work.
                    // assert false;
                }
            }
        }

2、Formatter  
JDK的介绍:

/**
     * A Formatter provides support for formatting LogRecords.
     * <p>
     * Typically each logging Handler will have a Formatter associated
     * with it.  The Formatter takes a LogRecord and converts it to
     * a string.
     * <p>
     * Some formatters (such as the XMLFormatter) need to wrap head
     * and tail strings around a set of formatted records. The getHeader
     * and getTail methods can be used to obtain these strings.
     *
     * @since 1.4
     */

Formatter支持格式化日志数据；通常每个Handler中都会有一个Formatter引用，Formatter可以将LogRecord对象转换为一个string字符串。

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