java 异常与记录日志

一. 你可能还想利用java.util.logging工具将输出记录到日志中

package exceptions;
//: exceptions/LoggingExceptions.java
// An exception that reports through a Logger.
import java.util.logging.*;
import java.io.*;

class LoggingException extends Exception {
  private static Logger logger =
    Logger.getLogger("LoggingException");//Logger.getLogger()方法创建了一个String参数相关联的Logger对象
                      //(通常是与错误相关的包名或类名),这个Logger对象会将其输出发送到System.err
  public LoggingException() {
    StringWriter trace = new StringWriter();
    printStackTrace(new PrintWriter(trace));//printStackTrace不会产生默认字符串,为了获取字符串,我们使用重载的
     //printStackTrace()方法,它接受一个java.io.PrintWriter()对象作为参数,如果我们将一个SrtingWriter对象传递给
     //这个Printwrite()构造器,那么就可以调用toString()方法,就可以抽取为一个String
    logger.severe(trace.toString());//severe写入日志
  }
}

public class LoggingExceptions {
  public static void main(String[] args) {
    try {
      throw new LoggingException();
    } catch(LoggingException e) {
      System.err.println("Caught " + e);
    }
    try {
      throw new LoggingException();
    } catch(LoggingException e) {
      System.err.println("Caught " + e);
    }
  }
} /* Output: (85% match)
Aug 30, 2005 4:02:31 PM LoggingException <init>
SEVERE: LoggingException
        at LoggingExceptions.main(LoggingExceptions.java:19)

Caught LoggingException
Aug 30, 2005 4:02:31 PM LoggingException <init>
SEVERE: LoggingException
        at LoggingExceptions.main(LoggingExceptions.java:24)

Caught LoggingException
*///:~

二. 尽管LoggingException将所有记录日志的基础设施都构建在了异常自身中,使用它非常方便.但更常见的是我们需要捕获和记录其它人编写的异常,

package exceptions;
//: exceptions/LoggingExceptions2.java
// Logging caught exceptions.
import java.util.logging.*;
import java.io.*;

public class LoggingExceptions2 {
  private static Logger logger =
    Logger.getLogger("LoggingExceptions2");
  static void logException(Exception e) {
    StringWriter trace = new StringWriter();
    e.printStackTrace(new PrintWriter(trace));
    logger.severe(trace.toString());
  }
  public static void main(String[] args) {
    try {
      throw new NullPointerException();
    } catch(NullPointerException e) {
      logException(e);
    }
  }
} /* Output: (90% match)
Aug 30, 2005 4:07:54 PM LoggingExceptions2 logException
SEVERE: java.lang.NullPointerException
        at LoggingExceptions2.main(LoggingExceptions2.java:16)
*///:~

我们还可以进一步自定义异常,比如加入额外的构造器和成员,因为我们必须在异常处理信息中生成日志信息,新的异常添加了字段x以及设定x值的构造器和读取数据,的方法,此外,还覆盖了getMassge()方法,以产生更详细的信息,对异常来说getMassge()方法有点类似于toString()方法

package exceptions;
//: exceptions/ExtraFeatures.java
// Further embellishment of exception classes.
import static net.mindview.util.Print.*;

class MyException2 extends Exception {
  private int x;
  public MyException2() {}
  public MyException2(String msg) { super(msg); }
  public MyException2(String msg, int x) {
    super(msg);
    this.x = x;
  }
  public int val() { return x; }
  public String getMessage() { //相当于toString()方法
    return "Detail Message: "+ x + " "+ super.getMessage();
  }
}

public class ExtraFeatures {
  public static void f() throws MyException2 {
    print("Throwing MyException2 from f()");
    throw new MyException2();
  }
  public static void g() throws MyException2 {
    print("Throwing MyException2 from g()");
    throw new MyException2("Originated in g()");
  }
  public static void h() throws MyException2 {
    print("Throwing MyException2 from h()");
    throw new MyException2("Originated in h()", 47);
  }
  public static void main(String[] args) {
    try {
      f();
    } catch(MyException2 e) {
      e.printStackTrace(System.out);
    }
    try {
      g();
    } catch(MyException2 e) {
      e.printStackTrace(System.out);
    }
    try {
      h();
    } catch(MyException2 e) {
      e.printStackTrace(System.out);
      System.out.println("e.val() = " + e.val());
    }
  }
} /* Output:
Throwing MyException2 from f()
MyException2: Detail Message: 0 null
        at ExtraFeatures.f(ExtraFeatures.java:22)
        at ExtraFeatures.main(ExtraFeatures.java:34)
Throwing MyException2 from g()
MyException2: Detail Message: 0 Originated in g()
        at ExtraFeatures.g(ExtraFeatures.java:26)
        at ExtraFeatures.main(ExtraFeatures.java:39)
Throwing MyException2 from h()
MyException2: Detail Message: 47 Originated in h()
        at ExtraFeatures.h(ExtraFeatures.java:30)
        at ExtraFeatures.main(ExtraFeatures.java:44)
e.val() = 47
*///:~