Summary of basic knowledge of exception handling

exception handling

Exception handling classification

• Error error
  Java Serious problems that virtual machines can't solve , Such as JVM System internal error 、 Serious situations such as resource exhaustion . Such as ：StackOverflowError Stack overflow and OutOfMemoryError Heap overflow OOM , Generally do not write targeted code for processing （ Exception handling code ）, But just make changes to the original code .
• Exception abnormal
  General problems caused by other programming errors or accidental external factors , You can use targeted code for processing , Do not change the original code , Only add exception preprocessing ：

Exception Exception Architecture

Compile time exception checked

• Runtime exception ： All are RuntimeException Class and its subclass exception , Such as NullPointerException( Null pointer exception )、IndexOutOfBoundsException( Subscript out of range exception ) etc. , These exceptions are not checked , The program can choose to capture the processing , Or not . These exceptions are usually caused by program logic errors , Programs should try to avoid such exceptions from a logical point of view .
• Runtime exceptions are characterized by Java The compiler will not check it , in other words , When this kind of exception may appear in the program , Even if it doesn't work try-catch Statement capture it , It doesn't work throws The clause declares to throw it , Will also compile through .

Runtime exception unchecked

• Non runtime exception （ Compile exception ）： yes RuntimeException An exception , They all belong to Exception Classes and subclasses . From the perspective of program syntax, it is an exception that must be handled , If you don't deal with , The program can't be compiled . Such as IOException、SQLException And user-defined Exception abnormal ,
• In general, no custom check exception .

exception handling

3 Exception handling statements

printStackTrance Is the most commonly used

e.toString()：   Get exception types and error messages 
e.getMessage(): Get the wrong message 
e.printStackTrace()： Print out the exception type on the console , Error information, error location, etc 

try-catch-finally

• catch There can be multiple , You can also put more than one catch Merged into a parent class catch
• finally It is not recommended to have return, But according to the situation , Here are some special return The situation of , Pay attention to the difference between basic data types and reference data types

try in	catch in	finally in	Return value situation ( Basic data type )	Return value situation ( Reference type )
Yes	nothing	Yes	return finally Stack value in	Same as left
Yes	Yes and abnormal ( perform )	Yes	perform finally, But return catch In the stack return value	perform finally And return to finally
nothing	Yes	Yes	If there is no abnormality finally in , If there is any abnormality, then catch in	Execute unconditionally and return finally Value

throws Throw exceptions

This treatment method does not really solve Exception, And if you call a method ,throws Will be thrown up layer by layer , Generally, when quoting methods defined by others , The method can be throws Coming out Exception Conduct try-catch Handle , This can be more flexible

throw Active throwing

throw new RuntimeException() , Sometimes in order to ensure the integrity of logic , You need to throw exceptions to prevent operations , And back to throw The exception is handed over to the interceptor

• throw Compile time exception (throw new Exception("")), The method is throws The corresponding compile time exception
• throw Runtime exception (throw new RuntimeException("")), The method does not need throws

for instance ：throw Runtime exception , no need thorws

class Student {
	int id;
	public Student(int id){
		if(id>0){
		this.id = id;
		}
		else{
			throw new RuntimeException("id It can't be negative ");
	}
	}}

Another example ：throw Compile time exception , need throws

class Student {
	int id;
	public Student(int id) throws Exception{
		if(id>0){
		this.id = id;
		}
		else{
			throw new Exception();
		}
	}
}

Custom exception classes

Custom exception classes need to meet

• Inherit from an exception class
• Null parameter construction and String msg structure
• Appoint serialVersionUID( Specify serialization version )

Example ：

class MyException extends RuntimeException{
	static final long serialVersionUID = -123456789L;
	
	public MyException(){}
	
	public MyException(String msg){
		super(msg);
	}
}

It is exactly the same as ordinary exception classes in use