SPRING BOOT EXCEPTION HANDLING

# Java & Spring Boot Exception Handling Notes

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## 1. What is Exception?
Exception = unwanted situation that breaks normal flow of program.

Goal of exception handling:
- Program crash na ho
- Error ko control karna
- Proper message dena

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## 2. Java Exception Hierarchy

Throwable
├── Error
└── Exception
├── RuntimeException (Unchecked)
└── Other Exceptions (Checked)

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## 3. Types of Exception (Java Level)

### (A) Checked Exception
- Compile time pe check hoti hai
- Handle karna compulsory hai (`try-catch` OR `throws`)
- Extends: `Exception` (but not RuntimeException)

Examples:
- IOException
- SQLException
- ClassNotFoundException
- FileNotFoundException
- InterruptedException

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### (B) Unchecked Exception
- Runtime pe aati hai
- Handle karna compulsory nahi
- Extends: `RuntimeException`

Examples:
- NullPointerException
- ArithmeticException
- ArrayIndexOutOfBoundsException
- NumberFormatException
- IllegalArgumentException

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### (C) Error
- JVM / system level problem
- Normally handle nahi karte
- Custom Error banana bad practice

Examples:
- OutOfMemoryError
- StackOverflowError

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## 4. Checked vs Unchecked Rule

extends Exception → Checked Exception
extends RuntimeException → Unchecked Exception
extends Error → Error (avoid)

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## 5. Can we extend other exceptions?

Technically YES:
- Can extend IOException, NullPointerException, etc.

But Best Practice:
- Extend only:
  - Exception (for checked)
  - RuntimeException (for unchecked)

Avoid:
- Extending NullPointerException, IOException, SQLException etc.

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## 6. try-catch vs throws

### try-catch:
- Exception yahin handle hoti hai

```java
try {
   FileReader fr = new FileReader("a.txt");
} catch (IOException e) {
   System.out.println("File error");
}

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throws:

• Caller ko responsibility deta hai

public void readFile() throws IOException {
   FileReader fr = new FileReader("a.txt");
}

Rule:
Checked Exception = try-catch OR throws (one compulsory)

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7. throw vs throws

• throw → manually exception throw karna
• throws → method signature me likhna

throw new IOException("File not found");

public void read() throws IOException { }

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8. Common Exception Meaning

ClassNotFoundException

• Jab runtime pe class nahi milti

FileNotFoundException

• Jab file nahi milti / path galat

IOException

• File / network / stream problem

SQLException

• SQL database problem (MySQL, PostgreSQL, Oracle)

Not used in MongoDB.

MongoDB uses:

• MongoException
• DuplicateKeyException

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InterruptedException

• Jab thread sleep/wait/join me ho
• Aur interrupt ho jaye

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9. Custom Exception

Checked Custom Exception

class MyCheckedException extends Exception {
}

Unchecked Custom Exception

class MyUncheckedException extends RuntimeException {
}

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10. Java vs Spring Boot Exception Handling

Java Level:

• try-catch
• throws
• method level handling

Spring Boot Level:

• Controller/API level handling
• Client ko JSON response dena

Uses:

• @ExceptionHandler
• @ControllerAdvice / @RestControllerAdvice

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11. Spring Boot Exception Handling Example

Custom Exception

class UserNotFoundException extends RuntimeException {
   public UserNotFoundException(String msg){
      super(msg);
   }
}

Global Handler

@RestControllerAdvice
class GlobalExceptionHandler {

   @ExceptionHandler(UserNotFoundException.class)
   public ResponseEntity<String> handle(UserNotFoundException e){
      return ResponseEntity.status(404).body(e.getMessage());
   }

   @ExceptionHandler(Exception.class)
   public ResponseEntity<String> handleAll(Exception e){
      return ResponseEntity.status(500).body("Server Error");
   }
}

Controller

@GetMapping("/user/{id}")
public User getUser(@PathVariable int id){
   return repo.findById(id)
     .orElseThrow(() -> new UserNotFoundException("User not found"));
}

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12. Key Interview Lines

• Checked exception = compile time, must handle
• Unchecked exception = runtime, optional handling
• SQLException only for SQL DB, not MongoDB

• Custom exception:

  • extends Exception → checked
  • extends RuntimeException → unchecked

• Spring Boot handles API errors using @ControllerAdvice

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13. Summary

Type	Extend	Handle
Checked	Exception	try-catch OR throws
Unchecked	RuntimeException	optional
Error	Error	avoid

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