Java - Exception 異常處理

exception architecture

• Error：程序無法處理的錯誤。
• Exception：程序本身可以處理的異常。
  • unchecked (RuntimeException): All exception types that are direct or indirect subclasses of RuntimeException are unchecked exceptions.
  • checked: Java compiler enforces special requirements for checked exceptions (discussed momentarily). An exception’s type determines whether it’s checked or unchecked.

	unchecked	checked
mandatory inspection	x	v
spring auto rollback	v	x
example exception	NullPointerException	FileNotFoundException

try-catch-finally

try-catch-finally 用法：

• try：用以捕獲異常，如果沒有catch塊，則必須跟一個finally塊
• catch：用以處理try捕獲的異常，可以接零個或多個catch塊
• finally：
  • 無論是否捕獲或處理異常，finally塊里的語句都會被執行
  • 當在try塊或catch塊遇到return語句時，finally語句將在方法返回之前執行，並且finally語句的返回值將會覆蓋原始的返回值

public static int f(int value) {
    try {
        return value * value;
    } finally {
        if (value == 2) {
            return 0;
        }
    }
}

如果調用f(2)，返回值將是0，而不是4，因為finally語句的返回值覆蓋了try語句塊的返回值

try-with-resources

后用先关，先用后关

override exception

• 父類別throws例外範圍 >= 子類別throws例外範圍
• 子類別可以丢IOException，但子類別不可以丢Exception，因為Exception > IOException

exception message

Get exception message:

1. public string getMessage(): simple message
2. public string toString(): detailed message
3. public string getLocalizedMessage:
   • override this method with a subclass of Throwable to generate localized information
   • if the subclass does not override the method, it is the same as the getMessage() message
4. public void printStackTrace(): print the exception information encapsulated by the Throwable object

Send a stacktrace to log4j: log.error( "failed!", e );

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