Does using braces inside a Java method reduce performance?

问题

It seems as though double-brace initialization increases overhead.

Does using braces inside of a method also reduce performance?

eg.

public class DoIReducePerformanceToo {

    public void aMethod() {

        {
           // Is it a bad idea to use these?
        }

    }

}

I've taken a look at Java's grammar and it seems that this is classified as a block:

Block: 
    { BlockStatements }

BlockStatements: 
    { BlockStatement }

BlockStatement:
    LocalVariableDeclarationStatement
    ClassOrInterfaceDeclaration
    [Identifier :] Statement

but I'm not sure where in the grammar double-brace initialization falls.

My question: does using block statements in methods reduce performance in Java? And are these blocks of the same nature as double-brace initialization?

EDIT:

Inner class instantation is:

ClassCreatorRest: Arguments [ClassBody]

ClassBody: 
    { { ClassBodyDeclaration } }

回答1:

The double-brace initialization trick has nothing to do with normal scopes.

Instead, it creates an anonymous class that inherits the type your initializing, and runs your code in an initialization block (which is syntactic sugar for a constructor).

This extra class has overhead.

回答2:

The block syntax is a part of the grammar, but also changes things such as variable scope. However, after compilation, variables, syntax, scope, and all is just converted into a plain bytecode format. The bytecode does not care about scoping rules, and the like, so there should be no overhead to using extra blocks in your code.

For example, the code

void something()
{
    int x = 5;
    randomStuffWithInt(x);
    {
        int x = 10;
        somethingWithInt(x);
    }
}

could be converted (alpha conversion) to

void something()
{
    int x = 5;
    randomStuffWithInt(x);
    int y = 10;
    somethingWithInt(y);
}

At runtime, it should be exactly the same speed.

回答3:

Late answer, but... I believe there will actually be an execution impact (though not necessarily a large one) given that the run-time engine will create a local variable symbol table on the stack to house variables within the block. If this is done in a looping context or in code otherwise requiring high speed, it may be advisable to avoid its usage.

来源：https://stackoverflow.com/questions/17558977/does-using-braces-inside-a-java-method-reduce-performance

标签

java

performance

block