返回如何在try，catch，最后在Java中工作？

Question

I can't understand exactly how return works in try , catch .

• If I have try and finally without catch , I can put return inside the try block.
• If I have try , catch , finally , I can't put return in the try block.
• If I have a catch block, I must put the return outside of the try , catch , finally blocks.
• If I delete the catch block and throw Exception , I can put the return inside the try block.

How do they work exactly? Why I can't put the return in the try block?

Code with try , catch , finally

 public int insertUser(UserBean user) {
     int status = 0;

     Connection myConn = null;
     PreparedStatement myStmt = null;

     try {
         // Get database connection
         myConn = dataSource.getConnection();

         // Create SQL query for insert
         String sql = "INSERT INTO user "
                    + "(user_name, name, password) "
                    + "VALUES (?, ?, ?)";

         myStmt = myConn.prepareStatement(sql);

         // Set the parameter values for the student
         myStmt.setString(1, user.getUsername());
         myStmt.setString(2, user.getName());
         myStmt.setString(3, user.getPassword());

         // Execute SQL insert
         myStmt.execute();
     } catch (Exception exc) {
         System.out.println(exc);
     } finally {
         // Clean up JDBC objects
         close(myConn, myStmt, null);
     }

     return status;
 }

Code with try , finally without catch

 public int insertUser(UserBean user) throws Exception {
     int status = 0;

     Connection myConn = null;
     PreparedStatement myStmt = null;

     try {
         // Get database connection
         myConn = dataSource.getConnection();

         // Create SQL query for insert
         String sql = "INSERT INTO user "
                    + "(user_name, name, password) "
                    + "VALUES (?, ?, ?)";

         myStmt = myConn.prepareStatement(sql);

         // Set the parameter values for the student
         myStmt.setString(1, user.getUsername());
         myStmt.setString(2, user.getName());
         myStmt.setString(3, user.getPassword());

         // Execute SQL insert
         myStmt.execute();

         return status;
     } finally {
         // Clean up JDBC objects
         close(myConn, myStmt, null);
     }
 }

Answer 1

Yes, it's confusing.

In Java, all program control paths of a non- void function must finish with a return , or throw an exception. That's the rule put nice and simply.

But, in an abomination, Java allows you to put an extra return in a finally block, which overrides any previously encountered return :

try {
    return foo; // This is evaluated...
} finally {
    return bar; // ...and so is this one, and the previous `return` is discarded
}

Answer 2

And if I have try, catch, finally I can't put return in the try block.

You absolutely can. You just need to make sure that every control path in your method is terminated properly. By that I mean: every execution path through your method either ends in a return , or in a throw .

For instance, the following works:

int foo() throws Exception { … }

int bar() throws Exception {
    try {
        final int i = foo();
        return i;
    } catch (Exception e) {
        System.out.println(e);
        throw e;
    } finally {
        System.out.println("finally");
    }
}

Here, you've got two possible execution paths:

1. final int i = foo()
2. either
   1. System.out.println("finally")
   2. return i
3. or
   1. System.out.println(e)
   2. System.out.println("finally")
   3. throw e

Path (1, 2) is taken if no exception is thrown by foo . Path (1, 3) is taken if an exception is thrown. Note how, in both cases, the finally block is executed before the method is left.

Answer 3

Finally block will always execute even if we caught the exception in catch block or even our try block executed as expected.

so when does finally block will be execute with in the flow...

if we have return statement inside the try/catch block then before executing the return statement finally block will be executed (like as for closing the connection or I/O)

function returnType process() {
  try {
      // some other statements
      // before returning someValue, finally block will be executed
      return someValue;
  } catch(Exception ex) {
     // some error logger statements
     // before returning someError, finally block will be executed
     return someError;
  } finally {
    // some connection/IO closing statements
    // if we have return inside the finally block
    // then it will override the return statement of try/catch block
    return overrideTryCatchValue;
  }
}

but if you have return statement inside the finally statement then it will override the return statement inside the try or catch block.

Answer 4

This is normal program flow when exception handling is involved. Having catch block in the code creates a case where code path can directly jump in to catch block. This defeats the mandate of having return statement in the method which returns something. It is possible that return statement may not get executed if exception occurs, hence compiler throws error. So to avoid this issue, you need at least 1 more return statement in a method.

If you have added a return statement in try-finally block and you dont have catch block, it is ok. There is no case of abnormal code path here.

If you have added a return statement in try block and you have catch block, then you can either add return in catch block or at the end of method.

If you have added a return statement in try block and you have catch block and finally block, then you can either add return in catch block or at the end of method. You can also choose to add return in finally block. If you are using eclipse, it will generate a warning which can be suppressed using below above method definition -

@SuppressWarnings("finally")

Answer 5

I think this is what you're asking:

And if I have try, catch, finally I can't put return in the try block.

So if you add a catch block, you can't put a return in the try block.

The problem is that if you add a catch then control drops through and you need a return at the end of the method or it's a syntax error. I haven't tested this but I assume you could put a return in the try block, but you would also have to add one inside the catch or at then end of the method as you have now.

Answer 6

In the second example, if the checked exception occurs then it hands over to calling a method.

In the first example, if the checked exception occurs then it handles in the same method, as catch block take the responsibility to handle the exception.

if you write return statement in catch block then it works.
ie,

try{  
    return ..  
}catch(Exception e){  
    return ..  
}finally{  
}

But it is not good programming practice.

Answer 7

使用void函数以外的公共函数时，应返回一些内容，否则函数不会返回。