Generally, all Apex code runs in system mode, where the permissions and record sharing of the current user are not taken into account. The system method runAs enables you to write test methods that change the user context to an existing user or a new user so that the user’s record sharing is enforced. The runAs method doesn’t enforce user permissions or field-level permissions, only record sharing.

Note: We can use runAs only in test methods. The original system context is started again after all runAs test methods complete. The runAs method ignores user license limits. We can create new users with runAs even if your organization has no additional user licenses.

The following items use the permissions granted by the user specified with runAs running as a specific user:

• dynamic Apex
• methods using with sharing or without sharing
• shared records

Example:

In the following example, a new test user is created, then code is run as that user, with that user’s record sharing access:

@isTest
private class TestRunAs {
   public static testMethod void testRunAs() {
      // Setup test data
      // This code runs as the system user
      Profile p = [SELECT Id FROM Profile WHERE Name='Standard Test User']; 
      User u = new User(Alias = 'standt', Email='standardtestuser@force.com', 
      EmailEncodingKey='UTF-8', LastName='TestUser', LanguageLocaleKey='en_US', 
      LocaleSidKey='en_US', ProfileId = p.Id, 
      TimeZoneSidKey='America/Los_Angeles', UserName='standardtestuser@force.com');

      System.runAs(u) {
         // The following code runs as user 'u' 
         System.debug('Current User: ' + UserInfo.getUserName());
         System.debug('Current Profile: ' + UserInfo.getProfileId()); 
      }
   }
}

Insert:

• Partial insert is not supported.
• Roll back is not supported.
• If we use the DML statement (Insert) in bulk operation, then if error occurs the execution will stop. In that case Apex code throws an error and none of the record will insert to the database.

Database.Insert

• Database methods are static methods available in Database class.
• Partial insert is supported.
• Roll back is supported.
• Includes the optional allorNone parameters that defaults true.
• If we use DML database methods (Database.Insert) in bulk operation, then if error occurs the remaining records will be inserted/updated means partial DML operation will be done. The only record throwing an error will not be inserted/updated.

Example: If we are inserting 10 records in an object, Where 5 records are correct and remaining 5 records are incorrect.

• In DML statement (Insert) all the 10 records will be failed, because if one record is incorrect or error means all other remaining records will not be inserted. It will throw error.
• In Database.insert 5 records will be inserted, remaining 5 records will be failed.(i.e. Partial DML Operation).

An exception is a special condition that changes the normal flow of program execution. That is, it’s when something bad happens that the program can’t deal with during execution. Exceptions are the language’s way of throwing up its hands and saying, I can’t deal with this, you need to handle those exceptions. we have three methods to catch this exceptions.

Apex allows to handle your exceptions, and write code to gracefully recover from an error. Apex uses the Try, Catch, Finally construct common to many other programming languages.

Try: If an exception occurs within the try block, that exception is handled by an exception handler associated with it. To associate an exception handler with a try block, you must put a catch block after it.
Catch: Each catch block is an exception handler that handles the type of exception indicated by its argument. The argument type, ExceptionType , declares the type of exception that the handler can handle and must be the name of a class that inherits from the Throwable class. The handler can refer to the exception with name.
Finally: This ensures that the finally block is executed even if an unexpected exception occurs. But finally is useful for more than just exception handling it allows the programmer to avoid having cleanup code accidentally bypassed by a return , continue , or break.

Simply You “try” to run your code. If there is an exception, you “catch” it and can run some code, then you can “finally” run some code whether you had an exception or not.

Here is an example of what these statements look like and the order in which they should be written:

try {
    // Perform some database operations that 
    //   might cause an exception.
} catch(DmlException e) {
    // DmlException handling code here.
} catch(Exception e) {
    // Generic exception handling code here.
} finally {
    // Perform some clean up.
}