Detailed Explanation of Exception Handling Mechanism in Java

I. Basic Concepts of Exception Handling
Exception handling is the core mechanism for dealing with runtime errors in Java programs. When unexpected situations occur, Java creates exception objects to describe error information and ensures the program does not crash through the exception handling mechanism.

II. Exception Classification System

1. The Throwable class is the superclass of all exceptions and errors.
   • Error: System-level errors (e.g., OutOfMemoryError) that programs cannot handle.
   • Exception: Exceptions that programs can handle, divided into two categories:
     • RuntimeException: Runtime exceptions (e.g., NullPointerException).
     • Non-RuntimeException: Checked exceptions (e.g., IOException).

III. Detailed Explanation of Exception Handling Keywords

1. try-catch-finally structure:

   try {
       // Code that may throw an exception
       FileInputStream fis = new FileInputStream("file.txt");
   } catch (FileNotFoundException e) {
       // Handle specific exception
       System.out.println("File not found");
   } catch (Exception e) {
       // Handle other exceptions
       System.out.println("Other exception");
   } finally {
       // Executes regardless of whether an exception occurred
       System.out.println("Clean up resources");
   }
   

2. throw keyword:

   • Used to actively throw an exception inside a method.

   void validateAge(int age) {
       if (age < 0) {
           throw new IllegalArgumentException("Age cannot be negative");
       }
   }
   

3. throws keyword:

   • Used in method declarations to indicate possible exception types that may be thrown.

   public void readFile() throws IOException {
       // Method may throw IOException
   }
   

IV. Best Practices for Exception Handling

1. Specific exception priority principle:

   // Not recommended
   catch (Exception e) { ... }
   
   // Recommended
   catch (FileNotFoundException e) { ... }
   catch (IOException e) { ... }
   

2. Use try-with-resources for resource management:

   // Resources are automatically closed, no finally block needed
   try (FileInputStream fis = new FileInputStream("file.txt");
        BufferedReader br = new BufferedReader(new InputStreamReader(fis))) {
       // Use resources
   } catch (IOException e) {
       // Handle exception
   }
   

3. Exception information propagation:

   catch (IOException e) {
       // Preserve original exception information
       throw new MyException("Error processing file", e);
   }
   

V. Custom Exception Implementation

1. Create a custom exception class:

   public class BusinessException extends Exception {
       private String errorCode;
   
       public BusinessException(String message, String errorCode) {
           super(message);
           this.errorCode = errorCode;
       }
   
       public String getErrorCode() {
           return errorCode;
       }
   }
   

2. Using custom exceptions:

   public void processOrder(Order order) throws BusinessException {
       if (order == null) {
           throw new BusinessException("Order cannot be null", "ORDER_001");
       }
       // Business logic processing
   }
   

VI. Performance Considerations for Exception Handling

1. Avoid using try-catch inside loops.
2. Exception instantiation has a high cost and should not be used for normal control flow.
3. Prefer using conditional checks to avoid exceptions.

By understanding the exception handling mechanism, you can write more robust and maintainable Java programs, effectively handling various runtime exception situations.