Chapter 7 exception handling

Catalog

  One 、 Exception Overview

​  Two 、 Exception Architecture

  3、 ... and 、 Exception handling ： Grab and throw the model

  Four 、 Exception handling mechanism 1 ：try-catch-finally

（1）try-catch-finally Use

（2）try-catch-finally in finally Use

5、 ... and 、 The second way of exception handling ：throws + Exception types

6、 ... and 、 Throw an exception manually

7、 ... and 、 How to customize exception class ？

8、 ... and 、 summary ： exception handling 5 Key words

One 、 Exception Overview

  Two 、 Exception Architecture

java.lang.Throwable
 (1)java.lang.Error: Generally do not write targeted code for processing
 (2)java.lang.Exception: Exception handling is possible
        ① Compile time exception (checked)
              IOException:FileNotFoundException
              ClassNotFoundException
        ② Runtime exception (unchecked)
              NullPointerException
              ArrayIndexOutOfBoundsException
              ClassCastException
              NumberFormatException
              InputMismatchException
              ArithmeticException
                                           Interview questions ： What are the common anomalies ？ Illustrate with examples

3、 ... and 、 Exception handling ： Grab and throw the model

Java Exception handling is provided Grab and throw the model .

Process one ：“ throw ”, During the normal execution of the program , Once something unusual happens , An object corresponding to the exception class will be generated at the exception code , And throw this object to Java Runtime system . Once the object is thrown , The subsequent code is no longer executed .      
   On the generation of abnormal objects ：① Exception objects generated automatically by the system
                                       ② Manually generate an exception object , And throw (throw)  
Process two ：“ Catch ”, It can be understood as an exception handling method ：①try-catch-finally   ②throws

Four 、 Exception handling mechanism 1 ：try-catch-finally

（1）try-catch-finally Use

  try{
          // There may be unexpected code
  }catch( Exception types 1 Variable name 1){
          // How to handle exceptions 1
  }catch( Exception types 2 Variable name 2){
          // How to handle exceptions 2
  }catch( Exception types 3 Variable name 3){
          // How to handle exceptions 3
  }
  ......
  finally{
          // Code that must execute
  }
   explain ：
  1.finally It's optional
  2. Use try Wrap up the code that may have an exception , In the process of execution , Once something unusual happens , An object corresponding to the exception class will be generated ,  According to the type of this object catch Match in
  3. once try The exception object in matches to a catch when , To get into catch Exception handling in . Once the process is complete , Just jump out of the current try-catch structure ( Without writing finally Under the circumstances ), Continue with the following code
  4.catch If the exception type in does not have a child parent relationship , Who declared on , It doesn't matter who declares that .catch If the exception type in satisfies the child parent relationship , It must be declared on the parent class . otherwise , Report errors
  5. Common ways to handle exception objects ：①String getMessage()   ②printStackTrace()
  6. stay try Variables declared in structure , There's something wrong with it try After the structure , Can't be called
  7.try-catch-finally Structures can be nested
 
  experience 1： Use try-catch-finally Structure handles compile time exceptions , So that the program will no longer report errors when compiling , But the runtime may still report an error . It's equivalent to we use try-catch-finally Structure will an exception that may occur at compile time , Delay to run time .
  experience 2： In development , Because runtime exceptions are common , So it is usually not written for runtime exceptions try-catch-finally It's structured .  But for compile time exceptions , Be sure to consider exception handling .

（2）try-catch-finally in finally Use

 1.finally It's optional
 2.finally What is declared in is code that must be executed . Even if catch There is another exception in ,try There is return structure ,catch There is return Statement, etc .
 3. Like database connection 、 I / O stream 、 Network programming Socket And so on ,JVM It can't be recycled automatically , We need to release resources manually . At this time, the resource release needs to be declared in finally in .

5、 ... and 、 The second way of exception handling ：throws + Exception types

  1."throws + Exception types " Write in the declaration of the method . Indicates when this method is executed , Exceptions that may be thrown .
         Once an exception occurs when the method body executes , An object of the exception class is still generated at the exception code , This object satisfies throws After the exception type , Will be thrown . Exception code and subsequent code will not be executed .
  2. experience ：
        try-catch-finally   Really handle the exception
        throws The only way to do this is to throw the exception to the caller of the method , It didn't really handle the exception .
  3. How to choose to use try-catch-finally still throws？
(1) If the overridden method in the parent class does not throws Handle exceptions in different ways , Then subclass overridden methods cannot be used throws, It means that if there is an exception in the method overridden by the subclass , You have to use try-catch-finally Method handling .
(2) Method of execution A in , Several other methods have been called , These methods are implemented by progressive relations . It is suggested that these methods be used throws The way to deal with , And the method of execution A Consider using try-catch-finally Method .

Add （ One of the rules of method rewriting ）： The exception thrown by the method overridden by the subclass is no larger than the exception thrown by the method overridden by the parent class

6、 ... and 、 Throw an exception manually

7、 ... and 、 How to customize exception class ？

  1. Inherits from the existing exception structure ：RuntimeException、Exception
  2. Provide global constants ：serialVersionUID
  3. Provide overloaded constructors

8、 ... and 、 summary ： exception handling 5 Key words