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Exceptions in Java

#Java基础35
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The document discusses exception handling in Java, categorizing exceptions into two main types: `Error`, which cannot be handled by the program, and `Exception`, which can be handled and is further divided into runtime exceptions and checked exceptions. It explains the execution of the `finally` block, which always runs unless certain conditions occur, such as an exception in the `finally` block or the program exiting. The document also clarifies the difference between `throw` and `throws`, and between `final`, `finally`, and `finalize`. Best practices for exception handling are emphasized, including avoiding the capture of runtime exceptions from the Java library, ensuring exceptions are handled properly, and being cautious with transaction rollbacks. It advises against using `return` in the `finally` block, as it can prevent the execution of the `try` block's return statement.

Exception Handling in Java#

Classification of Exceptions#

Java handles exceptions in an object-oriented manner, categorizing them by different types. The base class for all exception classes is the Throwable class, with two main subclasses: Error and Exception:

  • Error refers to errors that the program cannot handle, such as OOM, etc.
  • Exception refers to exceptions that the program can handle, divided into two major categories: RuntimeException and Checked Exception. Checked exceptions need to be handled using the try, catch, and finally keywords at compile time; otherwise, the compiler will throw an error.
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Execution of the finally Block#

The code block in finally will be executed regardless of whether an exception is caught. When involving break and continue statements, the finally clause will also be executed. In the following special cases, the finally block will not be executed:

  • An exception occurs in the finally block.
  • System.exit() is used in the preceding code or the program has already returned and exited.
  • The thread in which the program is running dies.
  • The CPU is shut down.

Execution of return and finally#

Reference: Java Basics - The Order of Execution of finally and return

  • The return statement is executed first, but it does not return immediately; instead, it executes the finally statement, and after the finally statement has finished executing, the return statement returns the result!
  • The return statement in the finally block will override the return in the try block.

What is the difference between throw and throws?#

  • throw is used within a method, while throws is used in the method declaration.
  • throw is followed by an exception object, while throws is followed by an exception type.
  • throw can only follow one exception object, while throws can declare multiple exception types at once.

What is the difference between final, finally, and finalize?#

  • final: It is a modifier. If it modifies a class, that class cannot be inherited; if it modifies a method or variable, it indicates that this method and variable cannot be changed and can only be used.
  • finally: It is the last part of try{} catch{} finally{}, indicating that it will be executed regardless of any situation. The finally part can be omitted, but if it exists, the code inside finally will definitely be executed.
  • finalize: It is a method of the Object class, called when the garbage collector executes on the object being collected.

Exception Handling#

Reference from Alibaba's "Java Specification Manual"

  • Do not catch runtime exception classes defined in the Java library that inherit from RuntimeException, such as IndexOutOfBoundsException / NullPointerException. These exceptions should be pre-checked by the programmer to ensure the robustness of the program.
  • Catching exceptions is for handling them; do not catch them and do nothing, discarding them.
  • If a try block is placed in transactional code, after catching an exception, if a rollback is needed, be sure to pay attention to manually rolling back the transaction.
  • Do not use return in the finally block; returning from the finally block ends the execution of the method, and the return statement in the try block will not be executed.
  • Accurately distinguish exception types to avoid using try-catch on large blocks of code and avoid throwing Exception.
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