Java Custom Exception Example(Java自定义异常案例)

转译自:https://examples.javacodegeeks.com
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In this example we will look briefly(短暂的) at the basics of Exception, in Java Programming Language. We will also see, how to create a custom Exception Class.

在这个案例中,我们将使用Java语言来简要介绍“异常”的基础知识。我们同样也能够了解如何创建一个自定义的Exception类。

1、Basics of Exception(异常的基础知识)

As per(按照) oracle docs, An exception is an event, which occurs during the execution of a program, that disrupts(扰乱) the normal flow of the program’s instructions(指令).

按照oracle的文档介绍,异常是一个在程序运行期间发生的事件,会扰乱正常程序流程的指令。

In laymen(非专业人员) terms, when a condition occurs, in which the routine(程序) is not sure how to proceed in an ordinary way it creates an object of exception and hands it over to the runtime system to find an appropriate handler for the exception object. In case, the runtime system does not find an appropriate handler in the call hierarchy the runtime system terminates.

用非专业的话来说,当满足异常发生条件时,程序不确定能以普通的方式进行时,它创建一个异常对象并递交给运行时系统,以查找处理异常对象的合适的处理程序。在这种情况下,运行时系统在运行时系统终止的调用层次结构中没有找到合适的处理程序。

The exceptions have java.lang.Throwable as their superclass.The three main categories of Exceptional conditions are :

异常以java.lang.Throwable类作为父类。异常主要有三大类:

(1)Error(represented by java.lang.Error and its sub-classes)

错误(以java.lang.Error类及其子类表示)

(2)Checked Exception(represented by direct subclasses of java.lang.Exception except java.lang.RuntimeException)

Checked异常(由java.lang.Exception除了java.lang.RuntimeException类的子类表示)

(3)Unchecked or Runtime Exceptions (represented by java.lang.RuntimeException and its sub-classes)

Unchecked或运行时异常(由java.lang.RuntimeException类及其子类表示)

Errors : Errors denote serious abnormal(不正常的) condition with the application like OutOfMemoryError or VirtualMachineError. Any reasonable application should not try to recover from such a condition.

错误表示严重的异常情况,如OutOfMemoryError(内存溢出)或VirtualMachineError(虚拟机错误)。任何理由都不能使应用程序从这种情况中恢复过来。

Runtime Exception/Unchecked Exception : These types of exceptions usually indicate programming errors like NullPointerException or IllegalArgumentException. The application may or may not choose to recover from the condition.

这些类型的异常通常表示程序错误,如NullPointerException(空指针异常)或IllegalArgumentException(参数异常)。应用程序可以选择从该类异常中恢复,也可以选择不恢复。

Checked Exception : An application is supposed to catch these types of exceptions and recover reasonably from them. Examples include FileNotFoundException and ParseException.

应用程序应该捕获这类异常并合理恢复。例如FileNotFoundException(文件未找到异常)和ParseException(解析异常)。

2、Creating custom Exception(创建自定义异常)

The first thing before creating a custom exception, the developer should be able to justify(证明) the creation(创建,n.). As per(按照) Java Docs, You should write your own exception classes if you answer yes to any of the following questions; otherwise, you can probably use someone else’s.

创建自定义异常前,开发者应为此(行为)找到依据。按照Java文档,如果你对以下任何一个问题回答yes,你应该编写自己的异常类;否则,你(最好)用别人的。

(1)Do you need an exception type that isn’t represented by those in the Java platform?

你需要一个Java平台中没有(定义)的异常类型吗?

(2)Would it help users if they could differentiate(区分) your exceptions from those thrown by classes written by other vendors(供应商)?

如果用户可以将你的异常与其他供应商编写的类抛出的异常区分开来,那么这会对用户有帮助吗?

(3)Does your code throw more than one related exception?

您的代码是否会抛出多个相关异常?

(4)If you use someone else’s exceptions, will users have access to those exceptions? A similar question is, should your package be independent and self-contained?

如果你使用其他人的异常,用户是否可以访问这些异常?类似的问题是,你的包应该是独立的、自包含的吗?

Now, that you are sure that you really do want a create a custom Exception class we will begin writing the actual program.To create a custom checked exception, we have to sub-class from the java.lang.Exception class. And that’s it! Yes, creating a custom exception in java is simple as that!

现在,您确信您确实想要创建一个自定义的异常类,我们将开始编写实际的程序。要创建一个自定义异常,我们必须从java.lang.Exception派生子类。是的,在java中创建一个自定义异常很简单!

public class CustomException extends Exception{}

Sometimes though, the CustomException object will have to be constructed from another exception. So its important that we create a constructor for such a scenario.

但有时,CustomException对象需要通过另一个异常来构造。因此,为这样的场景创建构造函数是很重要的。

So a more complete class would be as below :

因此,一个更完整的类应如下所示:

CustomException.java:

package com.javacodegeeks.examples.exception;
public class CustomException extends Exception
{
    private static final long serialVersionUID = 1997753363232807009L;
        public CustomException()
        {}
        public CustomException(String message)
        {
            super(message);
        }
        public CustomException(Throwable cause)
        {
            super(cause);
        }
        public CustomException(String message, Throwable cause)
        {
            super(message, cause);
        }
        public CustomException(String message, Throwable cause, 
                         boolean enableSuppression, boolean writableStackTrace)
        {
            super(message, cause, enableSuppression, writableStackTrace);
        }
}

2.1、Testing the custom Exception(测试自定义异常) :
CustomExceptionTest.java:

package com.javacodegeeks.examples.exception;
public class CustomExceptionTest
{
    public static void main(String[] args)
    {
    try
        {
          testException(null);
        }
        catch (CustomException e)
        {
          e.printStackTrace();
        }
    }
    public static void testException(String string) throws CustomException
    {
          if(string == null)
            throw new CustomException("The String value is null");
    }
}

Similarly, an unchecked exception can be created by sub-classing from the java.lang.RuntimeException Class.

类似地,unchecked异常可以通过java.lang.RuntimeException的子类来创建。

public class CustomException extends RuntimeException{...}

3、Points to note(注意事项)

(1)An unchecked exception should be preferred(首选) to a checked exception. This will help programmes to be free of unnecessary try..catch blocks.

相比一个unchecked异常,应首选checked异常。这有助于程序摆脱不必要的try..catch块。

(2)Exceptions are abnormal conditions, and as such should not be used for controlling the flow of execution of the programmes(as demonstrated by example below).

异常是异常情况,不应该用于程序的流程控制(如下面示例所示)。

try
{
    for (int i = 0;; i++)
    {
        System.out.println(args[i]);
    }
}
catch (ArrayIndexOutOfBoundsException e)
{
        // do nothing
}

(3)A catch block should not be empty as shown in the example above. Such exceptions are very difficult to track(跟踪), since there is no logging.

catch块不应该是空的,如上面的示例所示。由于没有日志记录,因此很难跟踪这些异常。

(4)The name of a custom exception class should end with Exception. It improves the readability of the code. Instead of naming the class InvalidSerial, the class should be named InvalidSerialException.

自定义异常类的名称应该以Exception结尾。它提高了代码的可读性。类不应该命名为InvalidSerial,而应该命名为InvalidSerialException。

(5)When using ARM/try-with-resources blocks, if exception gets suppressed by the try-with-resources statement, we can use Throwable#getSuppressed() method.

当使用ARM/try-with-resources块时,如果异常被try-with-resources语句抑制,我们可以使用Throwable# getrepression()方法。

4、Closing Words(结束语)

Here,we tried to understand the basics of exception and how to create a custom exception of our own. We learned the best practices while creating a custom exception class and also how to handle(处理) an exception in a reasonable manner.

在这里,我们尝试理解异常的基本原理,以及如何创建我们自己的自定义异常。我们在创建自定义异常类时学习了最佳实践,以及如何以合理的方式处理异常。

附录,相关资料1(可与上述转译内容互参)

Throwable是所有Java中所有异常类的父类,有两种子类:ErrorException

1、Error: 表示由JVM所侦测到的无法预期的错误,由于这是属于JVM层次的严重错误,导致JVM无法继续执行,这是不可捕捉到的,无法采取任何恢复的操作,只能显示错误信息。Error类体系描述了Java运行系统中的内部错误以及资源耗尽的情形,应用程序不应该抛出这种类型的对象(一般是由虚拟机抛出)。假如出现这种错误,除了尽力使程序安全退出外,在其他方面是无能为力的。

2、Exception: 表示可恢复的异常,这是可捕捉到的。Java提供了两类主要的异常:Runtime Exception和checked exception。

(2-1)checked 异常是经常遇到的,如:IO异常、SQL异常。对于这种异常,JAVA编译器强制要求我们对出现的这些异常进行catch。所以,面对这种异常只能编写catch块去处理。这类异常一般是外部错误,例如试图从文件尾后读取数据等,这并不是程序本身的错误,而是在应用环境中出现的外部错误。

(2-2)Runtime Exception也称运行时异常,我们可以不处理。当出现这样的异常时,总是由虚拟机接管。比如:我们从来没有人去处理过NullPointerException异常,它就是运行时异常,并且NullPointerException还是最常见的异常之一。Runtime Exception体系包括错误的类型转换、数组越界访问和试图访问空指针等等。如果出现Runtime Exception,那么一定是程序员的错误。例如:可以通过检查数组下标和数组边界来避免数组越界访问异常。

出现运行时异常后,系统会把异常一直往上层抛,一直遇到处理代码。如果到最上层还没有处理块,多线程程序就由Thread.run()抛出,这个线程就退出;如果是单线程就被main()抛出,整个程序也就退出了。运行时异常是Exception的子类,也有一般异常的特点,是可以被catch块处理的。只不过往往我们不对他处理罢了。如果不对运行时异常进行处理,那么出现运行时异常之后,要么是线程中止,要么是主程序终止。

如果不想终止线程或程序,则必须捕捉所有的运行时异常,决不让这个处理线程退出。队列里面出现异常数据了,正常的处理应该是把异常数据舍弃,然后记录日志。不应该由于异常数据而影响下面对正常数据的处理。但并不代表在所有的场景你都应该如此,如果在其它场景,遇到了一些错误,如果退出程序比较好,这时你就可以不太理会运行时异常,或者是通过对异常的处理显式的控制程序退出。异常处理的目标之一就是为了把程序从异常中恢复出来。

    原文作者:MrDcheng
    原文地址: https://www.jianshu.com/p/d5832a39298e
    本文转自网络文章,转载此文章仅为分享知识,如有侵权,请联系博主进行删除。
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