Java异常使用

Posted 2022-11-01 等待戈多儿

Java异常就是Java对程序运行时错误的处理。Java的所有异常都继承自java.lang.Throwable类。Java的异常处理框架如下：

在《EffectIve Java》（高效的Java）一书中关于异常有几点建议：

1、只针对异常的情况才使用异常

2、对可恢复的情况的使用受检异常，对编程错误使用运行时异常

3、避免不必要的使用受检的异常

4、优先使用标准的异常

5、抛出与抽象相对应的异常

6、每个方法抛出的异常都要有文档

//1.非受检异常,无抛出和捕获
    public static void main(String[] args) 
        testUncheckedException();
        System.out.println("next ...");
    
    public static void testUncheckedException()
        Integer.parseInt("I am not a number");
        System.out.println("then ...");
    

输出结果：

start ...
Exception in thread "main" java.lang.NumberFormatException: For input string: "I am not a number"
	at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
	at java.lang.Integer.parseInt(Integer.java:492)
	at java.lang.Integer.parseInt(Integer.java:527)
	at org.littleUtil.ExceptionDemo.testUncheckedException(ExceptionDemo.java:195)
	at org.littleUtil.ExceptionDemo.main(ExceptionDemo.java:191)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:147)

此段程序期望输出next ...和end ... 由于异常错误的发生，程序没有正常的进行和终止

//2.非受检异常,捕获异常，也可直接抛出，但会导致方法无法正常运行
    public static void main(String[] args)
        System.out.println("start ...");
        testUncheckedException2();

        try 
            testUncheckedException3();
        catch (Exception e)
            System.out.println(e.getMessage());
        
        System.out.println("end ...");
    
    public static void testUncheckedException2()
        try 
            Integer.parseInt("I am not a number");
        catch (Exception e)
            System.out.println(e.getMessage());
        
        System.out.println("next ... in 2");
    
    public static void testUncheckedException3() throws NumberFormatException
        Integer.parseInt("I am not a number");
        System.out.println("next ... in 3");
    

输出结果：

start ...
For input string: "I am not a number"
next ... in 2
For input string: "I am not a number"
end ...

//3.受检的异常
    public static void main(String[] args)
        System.out.println("start ...");
        try 
            testCheckedException();
        catch (IOException e)
            System.out.println(e.getMessage());
        
        System.out.println("end ...");

    
    public static void testCheckedException() throws IOException
        File file = new File("D://notfound.txt");
        InputStream inputStream = new FileInputStream(file);
        inputStream.close();
    

输出结果：

start ...
D:\\notfound.txt (系统找不到指定的文件。)
end ...

//4.finally里不应该出现return
    public static void main(String[] args) 
        System.out.println("start ...");
        System.out.println(testFinally1());
        System.out.println("end ...");
    
    public static String testFinally1()
        try 
            if (true)
                throw new NumberFormatException("数字转换异常");
            
        catch (NumberFormatException e)
            System.out.println(e.getMessage());
            throw e;
        finally 
            return "OK";
        

    

输出结果：

start ...
数字转换异常
OK
end ...

  //5.finally里不应该出现return
    public static void main(String[] args) 
        System.out.println("start ...");
        try 
            System.out.println(testFinally2());
        catch (NumberFormatException e)
            System.out.println("main:"+e.getMessage());
        
        System.out.println("end ...");

    
    public static String testFinally2()
        try 
            if (true)
                throw new NumberFormatException("数字转换异常");
            
        catch (NumberFormatException e)
            System.out.println(e.getMessage());
            throw e;
        finally 
            System.out.println("finally ...");
        
        return "OK";
    

输出结果：

start ...
数字转换异常
finally ...
main:数字转换异常
end ...

4和5可以得出结论：如果finally里出现了return则，catch里抛出的异常不会被上一层捕获到。

在java异常机制中finally语句块里的代码始终都会被执行，如果其中出现return则表明程序正常返回，上一层则不会察觉到该段程序的异常问题。