What is the difference between Dart's snapshot

问题:

I've been reading up on Dart snapshots, and they're frequently compared to Smalltalk images. But to me, they sound alot like Java bytecode.

For example:

"A Dart snapshot is just a binary serialization of the token stream, generated from parsing the code. A snapshot is not a "snapshot of a running program", it's generated before the tokens are turned into machine code. So, no program state is captured in a snapshot."

Plus they're cross-platform:

"The snapshot format itself is cross-platform meaning that it works between 32-bit, 64-bit machines and so forth. The format has been made so that it's quick to read into memory with a emphasis on minimizing extra work like pointer fixups."

Am I getting it wrong somewhere?

Sources:
What is the snapshot concept in dart?
http://www.infoq.com/articles/google-dart

回答1:

Snapshots contain the VM data structures representing the loaded script in a serialized form similar to Smalltalk images. To get a better understanding of what is contained in the snapshot, we should take a look at what the Dart VM creates as it reads the script:

• Library objects, referring to all top-level structures such as classes or top-level methods and variables.
• Class objects, containing all objects describing all methods and fields.
• Script and Tokenstream objects representing all loaded source code.
• String objects for all used identifiers and string constants in the source code.

This object graph is serialized into a file when generating a snapshot using a format that is architecture agnostic. This allows the Dart VM to deserialize this snapshot file on 32-bit or 64-bit machines and recreate all of the necessary internal VM data structures much quicker than reading the original scripts from a set of files (see John's answer).

To clarify John's answer a bit. The Dart VM does not parse ALL of the source code when generating the snapshot. It only needs to parse the top level of the sources to be able to extract class, method and field definitions as these are represented in the serialized graph. In particular method bodies are not parsed and as it is customary for a scripting language errors will be only reported once control reaches the particular method.

The purpose of Java bytecodes is entirely different as Ladicek points out. You could create a snapshot of the VM data structures in a JVM once the bytecodes are loaded to get a similar effect.

In short: The snapshot contains an efficient representation of all the data structures allocated on the Dart VM heap which are needed to start executing the script.

-Ivan

回答2:

A Dart snapshot is just a roll up of all source files that has been parsed ahead of time. A Dart snapshot is not similar to a Java bytecode file. A Java bytecode file consists of JVM machine code and is the product of a compile, link, and assembly (into JVM machine code) phase.

A Dart snapshot is a binary file of a Dart program and it's import/part source file dependencies that has been parsed into an abstract syntax tree and rolled into a single file. Executing a Dart snapshot allows for faster startup times because:

1. Only 1 file must be loaded from disk or off network. In contrast, a non-snapshot Dart program must be fetched, then any imported files must be fetched, and so on. Before each subsequent source file request can be made the previously fetched source file must be parsed to find out if it's referencing more source files. Imagine if your Dart program imported 10 libraries which consisted of 10 source files each. That means 110 I/O requests and parses that are done one after another.
2. The parsing has been done ahead of time. It's already known to be syntactically correct and ready to be compiled by the Dart VM.

HTH, John