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Null value means that the value is yet to be assigned or need not be assigned whereas empty value means that the blank value has already been assigned once. Null is the default for the null able columns that hasn't be assigned any value.
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Ans. Application is unable to insert a record as it violates a unique constraint.
The exception states the constraint and Table can be located by the Entity mapping. So I will go to the DB and will first check to which all columns the unique constraint applies. And then I will go and check the code and logs to see how come the duplicate column values were attempted to be inserted when they were not supposed to be.
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Ans. Unique constraint ensures that a columns or combination or columns are always unique in a table and hence doesn't allow null or duplicates to be entered for the column or combination of columns.
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Ans. Primary Key constraint means that the column(s) should be unique and doesn't allow null. So Primary key constraint implies unique and not null constraint too.
Unique constraint implies not null constraint too as allowing null would violate uniqueness on those columns.
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Ans. Garbage collection mechanism frees up the memory that is no longer needed. In languages like C / C++ the deallocation needs to be done explicitly by the programmer and hence any leniency may result in memory leak. Garbage collection in java ensures that all unused memory is reclaimed and hence there are no memory leaks.Moreover it relieves the programmer from the hassle of carefully releasing all memory.
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Ans. It's the way for synchronization wherein we can store version information in the table , so that if the same entity is updated by two transactions, the last to commit changes is informed of the conflict, and does not override the other transaction's work.
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Ans. This looks like the case for optimistic locking wherein hibernate suspects that the information in table was updated by some other transaction after the entity was loaded by current transaction.
One way is to have synchronized entity state and don't detach the entity. Other could be to merge the entity with the table record rather than just directly persisting the entity.
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Ans. Counter controlled repetitions are the loops that are controlled with the use of counters or the loops where the number of repetitions are known in advance. for loop in java is the counter controlled repetition.
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Ans. Yes, it's a project by confluent that provides in built mechanism for streaming records from bigdata data source to apache kafka message queues and vice versa. It provides a variety of source and sink connectors to achieve this.
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Ans. Source connectors are the connectors that are used to get information from the source whereas sink connectors are used to deploy information to the destination.
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Ans. synchronized is a keyword and a modifier. The synchronized keyword is used to indicate that a method can be accessed exclusively by one thread at a time.
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Ans. Convention over configuration is a way of software design used by frameworks that attempt to decrease the number of decisions that a developer is required to make without necessarily losing flexibility. The objective is to reduce the unnecessary configurations and follow conventions to accomplish that. For example - DB Entity and DB Table name conventionally should be same and hence shouldn't require configuring mapping between the two.
"Spring Boot" and "Ruby on Rails" have adopted this design principle.
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Ans. Yes, It may result in some loss of flexibility as the application has to follow all conventions. Moreover it can contradict with other design paradigms or principles used in the application or framework.
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Ans. Hibernate mapping configurations are used for mapping hibernate entities and corresponding DB Tables. Conventionally Entities and Table can share the same name and hence framework can provide implicit mapping instead of explicit mapping through configurations. Though it may result in little loss of flexibility in extreme cases but will work with all applications following the convention.
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Ans. It is used for intercepting the request. It's primary use is to implement security policy. All or specific request types can be intercepted and hence forwarded to authentication / authorization module so as to facilitate authorized requests to application.
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Ans. It introduces a layer of adaptations before it reaches the final and desired interface.
Moreover sometimes all requests are forwarded to adapter class. Some of such requests doesn't even require any sort of adaptions as they are qualified to call the final interface directly and introducing overheads.
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