Julia Data Format - JLD2
JLD2 saves and loads Julia data structures in a format comprising a subset of HDF5, without any dependency on the HDF5 C library. It typically outperforms the JLD package (sometimes by multiple orders of magnitude) and often outperforms Julia’s built-in serializer. While other HDF5 implementations supporting HDF5 File Format Specification Version 3.0 (i.e. libhdf5 1.10 or later) should be able to read the files that JLD2 produces, JLD2 is likely to be incapable of reading files created or modified by other HDF5 implementations. JLD2 does not aim to be backwards or forwards compatible with the JLD package.
Reading and writing data
A new interface: jldsave
jldsave
makes use of julia’s keyword argument syntax to store files, thus leveraging the parser and not having to rely on macros. To use it, write
x = 1
y = 2
z = 42
# The simplest case:
jldsave("example.jld2"; x, y, z)
# it is equivalent to
jldsave("example.jld2"; x=x, y=y, z=z)
# You can assign new names selectively
jldsave("example.jld2"; x, a=y, z)
# and if you want to confuse your future self and everyone else, do
jldsave("example.jld2"; z=x, x=y, y=z)
Compression and non-default IO types may be set via positional arguments.
save_object and load_object functions
If only a single object needs to stored and loaded from a file, one can use save_object
and load_object
functions.
#
JLD2.save_object
— Function
save_object(filename, x)
Stores an object x
in a new JLD2 file at filename
. If a file exists at this path, it will be overwritten.
Since the JLD2 format requires that all objects have a name, the object will be stored as single_stored_object
. If you want to store more than one object, use @save
macro, jldopen
or the FileIO API.
Example
To save the string hello
to the JLD2 file example.jld2:
hello = "world" save_object("example.jld2", hello)
#
JLD2.load_object
— Function
load_object(filename)
Returns the only available object from the JLD2 file filename
(The stored object name is inconsequential). If the file contains more than one or no objects, the function throws an ArgumentError
.
Example
To load the only object from the JLD2 file example.jld2:
hello = "world" save_object("example.jld2", hello) hello_loaded = load_object("example.jld2")
save and load functions
The save
and load
functions, provided by FileIO, provide a mechanism to read and write data from a JLD2 file. To use these functions, you may either write using FileIO
or using JLD2
. FileIO will determine the correct package automatically.
The save
function accepts an AbstractDict
yielding the key/value pairs, where the key is a string representing the name of the dataset and the value represents its contents:
using FileIO
save("example.jld2", Dict("hello" => "world", "foo" => :bar))
The save
function can also accept the dataset names and contents as arguments:
save("example.jld2", "hello", "world", "foo", :bar)
When using the save
function, the file extension must be .jld2
, since the extension .jld
currently belongs to the previous JLD package.
If called with a filename argument only, the load
function loads all datasets from the given file into a Dict:
load("example.jld2") # -> Dict{String,Any}("hello" => "world", "foo" => :bar)
If called with a single dataset name, load
returns the contents of that dataset from the file:
load("example.jld2", "hello") # -> "world"
If called with multiple dataset names, load
returns the contents of the given datasets as a tuple:
load("example.jld2", "hello", "foo") # -> ("world", :bar)
File interface
It is also possible to interact with JLD2 files using a file-like interface. The jldopen
function accepts a file name and an argument specifying how the file should be opened:
using JLD2
f = jldopen("example.jld2", "r") # open read-only (default)
f = jldopen("example.jld2", "r+") # open read/write, failing if no file exists
f = jldopen("example.jld2", "w") # open read/write, overwriting existing file
f = jldopen("example.jld2", "a+") # open read/write, preserving contents of existing file or creating a new file
Data can be written to the file using write(f, "name", data)
or f["name"] = data
, or read from the file using read(f, "name")
or f["name"]
. When you are done with the file, remember to call close(f)
.
Like open
, jldopen
also accepts a function as the first argument, permitting do
-block syntax:
jldopen("example.jld2", "w") do file
file["bigdata"] = randn(5)
end
Groups
It is possible to construct groups within a JLD2 file, which may or may not be useful for organizing your data. You can create groups explicitly:
jldopen("example.jld2", "w") do file
mygroup = JLD2.Group(file, "mygroup")
mygroup["mystuff"] = 42
end
or implicitly, by saving a variable with a name containing slashes as path delimiters:
jldopen("example.jld2", "w") do file
file["mygroup/mystuff"] = 42
end
# or save("example.jld2", "mygroup/mystuff", 42)
Both of these examples yield the same group structure, which you can see at the REPL:
julia> file = jldopen("example.jld2", "r") JLDFile /Users/simon/example.jld2 (read-only) └─📂 mygroup └─🔢 mystuff
Similarly, you can access groups directly:
jldopen("example.jld2", "r") do file
@assert file["mygroup"]["mystuff"] == 42
end
or using slashes as path delimiters:
@assert load("example.jld2", "mygroup/mystuff") == 42
Unpack.jl API
When additionally loading the UnPack.jl package, its @unpack
and @pack!
macros can be used to quickly save and load data from the file-like interface. Example:
using UnPack
file = jldopen("example.jld2", "w")
x, y = rand(2)
@pack! file = x, y # equivalent to file["x"] = x; file["y"] = y
@unpack x, y = file # equivalent to x = file["x"]; y = file["y"]
The group file_group = Group(file, "mygroup")
can be accessed with the same file-like interface as the "full" struct.
Gotchas
Objects are cached during loading
JLD2 caches objects during loading. It may give you the same object twice. This can lead to surprising results if you edit loaded arrays. Note, the underlying file is not being edited!
julia> jldsave("demo.jld2", a=zeros(2)) julia> f = jldopen("demo.jld2") JLDFile /home/isensee/demo.jld2 (read-only) └─🔢 a julia> a = f["a"] # bind loaded array to name `a` 2-element Vector{Float64}: 0.0 0.0 julia> a[1] = 42; # editing the underlying array julia> f["a"] 2-element Vector{Float64}: 42.0 0.0 julia> a=nothing # remove all references to the loaded array julia> GC.gc(true) # call GC to remove the cache julia> f["a"] # a new copy is loaded from the file 2-element Vector{Float64}: 0.0 0.0
Cross-compatibility
JLD2 tries to write files in a way that allows you to load them on different operating systems and in particular both on 32bit and 64bit systems. However, many julia structs may be inherently different on different architectures making this task impossible. In particular, moving data from a 64bit system to a 32bit system is only guaranteed to work for basic datatypes.
Security
Beware of opening JLD2 files from untrusted sources. A malicious file may execute code on your computer. See e.g. here. To check a file, you can use JLD2DebugTools.jl to view what kinds of objects are stored.