java 异常链

1.) 常常会想要在捕获一个异常后抛出另一个异常,并且希望把原始异常的信息保存下来,被称为异常链.

2.)Throwable子类在构造器中可以接受一个cause(因由)对象作为参数.这个cause就是用来表示原始异常,这样通过原是异常传递给新的异常,使得即使在当前位置抛出了新的异常,也能通过异常链追踪到最初发生的位置

3.)三种基本异常链:

• Error(用于Java虚拟机报告系统错误)
• Exception
• RuntimeException

package exceptions;
//: exceptions/DynamicFields.java
// A Class that dynamically adds fields to itself.
// Demonstrates exception chaining.
import static net.mindview.util.Print.*;

class DynamicFieldsException extends Exception {}

public class DynamicFields {
  private Object[][] fields;
  public DynamicFields(int initialSize) {
    fields = new Object[initialSize][2];
    for(int i = 0; i < initialSize; i++)
      fields[i] = new Object[] { null, null };
  }
  public String toString() {
    StringBuilder result = new StringBuilder();
    for(Object[] obj : fields) {
      result.append(obj[0]);
      result.append(": ");
      result.append(obj[1]);
      result.append("\n");
    }
    return result.toString();
  }
  private int hasField(String id) {
    for(int i = 0; i < fields.length; i++)
      if(id.equals(fields[i][0]))
        return i;
    return -1;
  }
  private int
  getFieldNumber(String id) throws NoSuchFieldException {
    int fieldNum = hasField(id);
    if(fieldNum == -1)
      throw new NoSuchFieldException();
    return fieldNum;
  }
  private int makeField(String id) {
    for(int i = 0; i < fields.length; i++)
      if(fields[i][0] == null) {
        fields[i][0] = id;
        return i;
      }
    // No empty fields. Add one:
    Object[][] tmp = new Object[fields.length + 1][2];
    for(int i = 0; i < fields.length; i++)
      tmp[i] = fields[i];
    for(int i = fields.length; i < tmp.length; i++)
      tmp[i] = new Object[] { null, null };
    fields = tmp;
    // Recursive call with expanded fields:
    return makeField(id);
  }
  public Object
  getField(String id) throws NoSuchFieldException {
    return fields[getFieldNumber(id)][1];
  }
  public Object setField(String id, Object value)
  throws DynamicFieldsException {
    if(value == null) {
      // Most exceptions don't have a "cause" constructor.
      // In these cases you must use initCause(),
      // available in all Throwable subclasses.
      DynamicFieldsException dfe =
        new DynamicFieldsException();
      dfe.initCause(new NullPointerException());//除了三种基本异常类,其它类型的异常应该使用initCause()方法
      throw dfe;
    }
    int fieldNumber = hasField(id);
    if(fieldNumber == -1)
      fieldNumber = makeField(id);
    Object result = null;
    try {
      result = getField(id); // Get old value
    } catch(NoSuchFieldException e) {
      // Use constructor that takes "cause":
      throw new RuntimeException(e);//原始异常传递给了新的异常
    }
    fields[fieldNumber][1] = value;
    return result;
  }
  public static void main(String[] args) {
    DynamicFields df = new DynamicFields(3);
    print(df);
    try {
      df.setField("d", "A value for d");
      df.setField("number", 47);
      df.setField("number2", 48);
      print(df);
      df.setField("d", "A new value for d");
      df.setField("number3", 11);
      print("df: " + df);
      print("df.getField(\"d\") : " + df.getField("d"));
      Object field = df.setField("d", null); // Exception
    } catch(NoSuchFieldException e) {
      e.printStackTrace(System.out);
    } catch(DynamicFieldsException e) {
      e.printStackTrace(System.out);
    }
  }
} /* Output:
null: null
null: null
null: null

d: A value for d
number: 47
number2: 48

df: d: A new value for d
number: 47
number2: 48
number3: 11

df.getField("d") : A new value for d
DynamicFieldsException
        at DynamicFields.setField(DynamicFields.java:64)
        at DynamicFields.main(DynamicFields.java:94)
Caused by: java.lang.NullPointerException  //这里通过异常链显示了前一个异常
        at DynamicFields.setField(DynamicFields.java:66)
        ... 1 more
*///:~