java 異常與記錄日志

  一. 你可能還想利用java.util.logging工具將輸出記錄到日志中

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()方法創建了一個String參數相關聯的Logger對象
                      //(通常是與錯誤相關的包名或類名),這個Logger對象會將其輸出發送到System.err
  public LoggingException() {
    StringWriter trace = new StringWriter(); 
    printStackTrace(new PrintWriter(trace));//printStackTrace不會產生默認字符串,為了獲取字符串,我們使用重載的
     //printStackTrace()方法,它接受一個java.io.PrintWriter()對象作為參數,如果我們將一個SrtingWriter對象傳遞給
     //這個Printwrite()構造器,那么就可以調用toString()方法,就可以抽取為一個String
    logger.severe(trace.toString());//severe寫入日志
  }
}

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. 盡管LoggingException將所有記錄日志的基礎設施都構建在了異常自身中,使用它非常方便.但更常見的是我們需要捕獲和記錄其它人編寫的異常,

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)
*///:~

           我們還可以進一步自定義異常,比如加入額外的構造器和成員,因為我們必須在異常處理信息中生成日志信息,新的異常添加了字段x以及設定x值的構造器和讀取數據,的方法,此外,還覆蓋了getMassge()方法,以產生更詳細的信息,對異常來說getMassge()方法有點類似於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() { //相當於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
*///:~