Continuing from the previous blog post: Java Improvement Chapter (16)-----Exception (1)

       Java does provide us with a lot of exceptions, but the exception system cannot foresee all the errors that we want to report, so Java allows us to customize exceptions. It represents specific problems that may be encountered in the program. In a word: we don't have to stick to the existing exception types in Java.

The use of Java custom exceptions requires the following four steps:

       1. Define a class that inherits Throwable or its subclass.

​ 2. Add a constructor (of course, you can also use the default constructor without adding it).

         3. Throw this exception in a certain method class.

4. Catch the exception.

##

/** 自定义异常 继承Exception类 **/
public class MyException extends Exception{
    public MyException(){
        
    }
    
    public MyException(String message){
        super(message);
    }
}

public class Test {
    public void display(int i) throws MyException{
        if(i == 0){
            throw new MyException("该值不能为0.......");
        }
        else{
            System.out.println( i / 2);
        }
    }
    
    public static void main(String[] args) {
        Test test = new Test();
        try {
            test.display(0);
            System.out.println("---------------------");
        } catch (MyException e) {
            e.printStackTrace();
        }
    }
}

#Run result:

#                                                       

          There is a design pattern called the chain of responsibility pattern, which links multiple objects into a chain. The client The request is passed along this chain until it is received and processed. Similarly, the Java exception mechanism also provides such a chain: exception chain.

We know that every time we encounter an exception message, we need to try...catch, one is okay , what if multiple exceptions occur? Classification processing will definitely be more troublesome, so use one Exception to solve all exceptions. This is indeed possible, but this approach will inevitably make subsequent maintenance more difficult. The best way is to encapsulate these exception information and then capture our encapsulation class.

It is true that in applications, we sometimes not only need to encapsulate exceptions, but also need to pass them on. How to deliver? throws! binge, correct! ! But if you just use throws to throw exceptions, what should you do with your encapsulated class? ? We have two ways to handle exceptions. One is to throw it to the superior for processing, and the other is to try...catch does specific processing. But how does this relate to the above? We don't need to do any processing in the catch block of try...catch. We just use the throw keyword to actively throw the exception information we encapsulate. Then continue to throw the method exception through the keyword throws. Its upper layer can also perform such processing, and by analogy, an exception chain composed of exceptions will be generated.

##​​​​

同理，我们有时候在捕获一个异常后抛出另一个异常信息，并且希望将原始的异常信息也保持起来，这个时候也需要使用异常链。

在异常链的使用中，throw抛出的是一个新的异常信息，这样势必会导致原有的异常信息丢失，如何保持？在Throwable及其子类中的构造器中都可以接受一个cause参数，该参数保存了原有的异常信息，通过getCause()就可以获取该原始异常信息。

语法：

public void test() throws XxxException{
        try {
            //do something:可能抛出异常信息的代码块
        } catch (Exception e) {
            throw new XxxException(e);
        }
    }

示例：

public class Test {
    public void f() throws MyException{
         try {
            FileReader reader = new FileReader("G:\\myfile\\struts.txt");  
             Scanner in = new Scanner(reader);  
             System.out.println(in.next());
        } catch (FileNotFoundException e) {
            //e 保存异常信息
            throw new MyException("文件没有找到--01",e);
        }  
    }
    
    public void g() throws MyException{
        try {
            f();
        } catch (MyException e) {
            //e 保存异常信息
            throw new MyException("文件没有找到--02",e);
        }
    }
    
    public static void main(String[] args) {
        Test t = new Test();
        try {
            t.g();
        } catch (MyException e) {
            e.printStackTrace();
        }
    }
}

运行结果:

com.test9.MyException: 文件没有找到--02
    at com.test9.Test.g(Test.java:31)
    at com.test9.Test.main(Test.java:38)
Caused by: com.test9.MyException: 文件没有找到--01
    at com.test9.Test.f(Test.java:22)
    at com.test9.Test.g(Test.java:28)
    ... 1 more
Caused by: java.io.FileNotFoundException: G:\myfile\struts.txt (系统找不到指定的路径。)
    at java.io.FileInputStream.open(Native Method)
    at java.io.FileInputStream.<init>(FileInputStream.java:106)
    at java.io.FileInputStream.<init>(FileInputStream.java:66)
    at java.io.FileReader.<init>(FileReader.java:41)
    at com.test9.Test.f(Test.java:17)
    ... 2 more

如果在程序中,去掉e，也就是：throw new MyException("文件没有找到--02");

那么异常信息就保存不了，运行结果如下：

com.test9.MyException: 文件没有找到--02
    at com.test9.Test.g(Test.java:31)
    at com.test9.Test.main(Test.java:38)

PS:其实对于异常链鄙人使用的也不是很多，理解的不是很清楚，望各位指正！！！！

七、异常的使用误区

首先我们先看如下示例：该实例能够反映java异常的不正确使用（其实这也是我刚刚学Java时写的代码）！！

OutputStreamWriter out = null;
        java.sql.Connection conn = null;
        try {            //   ---------1
            Statement stat = conn.createStatement();
            ResultSet rs = stat.executeQuery("select *from user");
            while (rs.next()){
                out.println("name:" + rs.getString("name") + "sex:"
                        + rs.getString("sex"));
            }
            conn.close();         //------2
            out.close();
        } 
        catch (Exception ex){    //------3
            ex.printStackTrace();    //------4
        }

1、-----------1

对于这个try…catch块，我想他的真正目的是捕获SQL的异常，但是这个try块是不是包含了太多的信息了。这是我们为了偷懒而养成的代码坏习惯。有些人喜欢将一大块的代码全部包含在一个try块里面，因为这样省事，反正有异常它就会抛出，而不愿意花时间来分析这个大代码块有那几块会产生异常，产生什么类型的异常，反正就是一篓子全部搞定。这就想我们出去旅游将所有的东西全部装进一个箱子里面，而不是分类来装，虽不知装进去容易，找出来难啊！！！所有对于一个异常块，我们应该仔细分清楚每块的抛出异常，因为一个大代码块有太多的地方会出现异常了。

结论一：尽可能的减小try块！！！

2、--------2

What did you find here? Exceptions change the running process! ! What’s good is that exceptions change the program’s running flow. If an exception occurs in the program, conn.close();out.close(); is impossible to execute, which will inevitably result in resources not being released. Therefore, if the program uses resources such as files, Sockets, and JDBC connections, even if an exception is encountered, we must ensure that the occupied resources can be released correctly. This is where finally comes in: no matter whether an exception occurs or not, finally always has a chance to run, so finally is perfect for releasing resources.

         Conclusion 2: Ensure that all resources are released correctly. Make full use of the finally keyword.

#                         ----------3

//使用throws抛出异常
    public void f() throws MyException{
         try {
            FileReader reader = new FileReader("G:\\myfile\\struts.txt");  
             Scanner in = new Scanner(reader);  
             System.out.println(in.next());
        } catch (FileNotFoundException e) {
            throw new MyException("文件没有找到", e);    //throw
        }  
        
    }