Java Hack: Double Brace Initialization

Post by duncanj » Thu, Jul 16, 2009, 9:07 PM UTC

Post by jrockway » Thu, Jul 16, 2009, 9:24 PM UTC

ZitchDog wrote: post #3

Watch out - double-brace initialization is cool looking, but it's orders of magnitude slower than regular initialization as it must generate an anonymous class. Also, the generated class is put into the permgen space, which is not garbage collected. The permgen is pretty small by default - and if you fill it up, your system is hosed. I use them all the time in unit tests, but never in production code.

    my $class = Some::Meta::Class->create_anon_class(
        superclasses => ['Foo'],
        roles        => [qw/Commonly Applied Traits/],
    );

Post by scotth » Thu, Jul 16, 2009, 10:03 PM UTC

ZitchDog wrote: post #3

Watch out - double-brace initialization is cool looking, but it's orders of magnitude slower than regular initialization as it must generate an anonymous class. Also, the generated class is put into the permgen space, which is not garbage collected. The permgen is pretty small by default - and if you fill it up, your system is hosed. I use them all the time in unit tests, but never in production code.

     new ArrayList() {
       {}
     };
     new ArrayList();

     0  new Main$1 [16]
     3  invokespecial Main$1() [18]
     6  new java.util.ArrayList [19]
     9  invokespecial java.util.ArrayList() [21]

    class Main$1 extends java.util.ArrayList {

    // Method descriptor #6 ()V
    // Stack: 1, Locals: 1
    Main$1();
      0  aload_0 [this]
      1  invokespecial java.util.ArrayList() [8]
      4  return
    Line numbers:
      [pc: 0, line: 10]
      [pc: 4, line: 1]
    Local variable table:
      [pc: 0, pc: 5] local: this index: 0 type: new Main(){}

Post by elcron » Thu, Jul 16, 2009, 10:17 PM UTC

scotth wrote: post #6

ZitchDog wrote: post #3

Watch out - double-brace initialization is cool looking, but it's orders of magnitude slower than regular initialization as it must generate an anonymous class. Also, the generated class is put into the permgen space, which is not garbage collected. The permgen is pretty small by default - and if you fill it up, your system is hosed. I use them all the time in unit tests, but never in production code.

The compiler generates the anonymous class. To the runtime, isn't it just a class with a name given to it by the compiler, not a developer (question mark...my key broke today) It's true that it uses up permgen though, but if it only adds an initializer we're not talking about much space. Edit: I did an experiment. This is the code: new ArrayList () { {} }; new ArrayList (); Here is the bytecode: 0 new Main$1 [16] 3 i…

Post by jrockway » Fri, Jul 17, 2009, 1:53 AM UTC

sunkencity wrote: post #5

jrockway wrote: post #4

Earlier quoted context omitted.

Interesting. Even in Perl we cache and garbage-collect anon classes. (The caching is for the very common case of creating a subclass with optional roles applied: my $class = Some::Meta::Class->create_anon_class( superclasses => ['Foo'], roles => [qw/Commonly Applied Traits/], ); There is almost no speed hit the second time you create an instance of "Foo" with "Commonly", "Applied", and "Traits" applied. Not sure this…

perl has a more stable vm than java :) I like this trick though, it will help me with my seaside clone in java :)