There are a number of ways we might represent a lambda expression in bytecode, such as inner classes, method handles, dynamic proxies, and others. Each of these approaches has pros and cons. In selecting a strategy, there are two competing goals: maximizing flexibility for future optimization by not committing to a specific strategy, vs providing stability in the classfile representation. We can achieve both of these goals by using the invokedynamic feature from JSR 292 to separate the binary representation of lambda creation in the bytecode from the mechanics of evaluating the lambda expression at runtime. Instead of generating bytecode to create the object that implements the lambda expression (such as calling a constructor for an inner class), we describe a recipe for constructing the lambda, and delegate the actual construction to the language runtime. That recipe is encoded in the static and dynamic argument lists of an invokedynamic instruction.

The use of invokedynamic lets us defer the selection of a translation strategy until run time. The runtime implementation is free to select a strategy dynamically to evaluate the lambda expression. The runtime implementation choice is hidden behind a standardized (i.e., part of the platform specification) API for lambda construction, so that the static compiler can emit calls to this API, and JRE implementations can choose their preferred implementation strategy. The invokedynamic mechanics allow this to be done without the performance costs that this late binding approach might otherwise impose.

When the compiler encounters a lambda expression, it first lowers (desugars) the lambda body into a method whose argument list and return type match that of the lambda expression, possibly with some additional arguments (for values captured from the lexical scope, if any.) At the point at which the lambda expression would be captured, it generates an invokedynamic call site, which, when invoked, returns an instance of the functional interface to which the lambda is being converted. This call site is called the lambda factory for a given lambda. The dynamic arguments to the lambda factory are the values captured from the lexical scope. The bootstrap method of the lambda factory is a standardized method in the Java language runtime library, called the lambda metafactory. The static bootstrap arguments capture information known about the lambda at compile time (the functional interface to which it will be converted, a method handle for the desugared lambda body, information about whether the SAM type is serializable, etc.)

Method references are treated the same way as lambda expressions, except that most method references do not need to be desugared into a new method; we can simply load a constant method handle for the referenced method and pass that to the metafactory.