I talked about exception handling for more than half an hour during the interview yesterday

The strong wind blew away the haze of the past , The sun shines through the window , Through the window, you can see the mountains in the distance and the blue sky , It's much better than my rental house where I'll never see the sun . The sound of footsteps coming in interrupted my thoughts , A little sister sat in front of her , The smell of sweet perfume immediately went into my nostrils. .

Said the little sister with a smile ：” Hello! , I'm the interviewer today , So here we go ？“

I put away my straight eyes , say ：“ well .”

Miss said ：“ stay Java There are two major elements in exception handling ： Throw and catch exceptions . So what are the two types of exceptions thrown ？”

I answered immediately ：

Throwing exceptions can be divided into explicit and implicit . Explicit exception throwing is used in code throw Keyword to manually throw an exception instance . Implicit throwing exception is Java The virtual machine is executing , Encountered an exception that cannot continue execution , Automatically throw exception instances , For example, we often encounter null pointer exceptions （NullPointerException）.

Miss said ：“ very good , So what keywords are often used in catching exceptions ？”

I answered immediately ：

We usually use try、catch、finally Other key words .try It is used to mark the code that needs exception monitoring ;catch Used to capture try An exception of a specified type that is triggered in the monitored code , You can also define how to handle this exception type ;finally Used to declare a piece of code that must run no matter what exception happens , It avoids skipping some critical cleanup code , such as ： Close the opened IO resources .

Miss said ：“ very good , If three keywords are used together , What is the order of code execution ？”

I answered immediately ：

Under normal execution , Execute first try Execute the code in finally The code in .

If try The code in triggers an exception , And the exception is not caught ,finally The code will be executed directly , And throw the exception again after execution ; If the exception is catch Capture , Execute first catch Execute the code in finally The code in .

If catch The code in also triggers an exception , that finally The code will also be executed , And throw out catch The exception triggered by the code . If finally The code in also triggers an exception , Then it will interrupt the current finally Execution of code , And throw an exception .

Miss said ：“ very good , stay Java In the virtual machine , How to implement exception handling ？”

This question is a little difficult , I thought a little and replied ：

Mainly through Anomaly table . In the bytecode generated by compilation , Each method comes with an exception table . There may be multiple records in the exception table , Every record includes from The pointer 、to The pointer 、target Pointer and the type of exception caught . The values of these Pointers are bytecode indexes （bytecode index）, Used to locate bytecode .

among ,from Pointers and to The pointer indicates the scope of exception handling monitoring , such as ： try Coverage .target The pointer points to the starting position of the exception handling code , such as ：catch The starting position of the code .

If an exception is triggered ,Java The virtual opportunity traverses all records in the exception table from top to bottom . When the index value of the bytecode triggering the exception is within the monitoring range recorded in an exception table ,Java The virtual opportunity determines whether the thrown exception matches the exception that the record wants to catch . If the match , The control flow will be moved to this record target The bytecode to which the pointer points .

Miss said ：“ A bit abstract , Can you give me an example ？”

“ Certainly. .” I said , While writing on the paper ：

Log in and copy

public class OneMore {

    public static void main(String[] args) {
        String str = " Wanmao society ";
        try {
            str = "try";
        } catch (Exception e) {
            str = "catch";
        }
    }
}

     

      
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Of this code main In the method , I defined a paragraph try and catch Code . In the compiled bytecode , The exception table of this method has a record ：

Log in and copy

public static void main(java.lang.String[]);
  descriptor: ([Ljava/lang/String;)V
  flags: ACC_PUBLIC, ACC_STATIC
  Code:
    stack=1, locals=3, args_size=1
       0: ldc           #2                  // String  Wanmao society 
       2: astore_1
       3: ldc           #3                  // String try
       5: astore_1
       6: goto          13
       9: astore_2
      10: ldc           #5                  // String catch
      12: astore_1
      13: return
    Exception table:
       from    to  target type
           3     6     9   Class java/lang/Exception

     

      
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Its from Pointers and to The pointers are 3 and 6, Represents its monitoring range from index to 3 The bytecode starts , To index as 6 End of bytecode （ barring 6）. It should be recorded target Pointer is 9, Represents the exception handling from the index to 9 The bytecode starts . The last column of the record , Represents that the exception type caught by this exception handling is Exception.

When the index value of the bytecode triggering the exception is 3 and 6 Between time ,Java When the virtual opportunity judges whether the thrown exception is Exception. If it is , The control flow will be moved to the index 9 The bytecode of begins to execute .

The little sister said with satisfaction ：“ very good , I'll call HR Talk to you , With luck , You can send it today offer.”

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reference ：
《Java Programming idea 》
《Java The core technology 》
《 In depth understanding of Java virtual machine ：JVM Advanced features and Practice 》