JLD2
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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
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)
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)
In the above examples, ;
after the filename is important. Compression and non-default IO types may be set via positional arguments.
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
Custom Serialization
The API is simple enough, to enable custom serialization for your type A
you define
a new type e.g. ASerialization
that contains the fields you want to store and define
JLD2.writeas(::Type{A}) = ASerialization
.
Internally JLD2 will call Base.convert
when writing and loading, so you need to make sure to extend that for your type.
struct A
x::Int
end
struct ASerialization
x::Vector{Int}
end
JLD2.writeas(::Type{A}) = ASerialization
Base.convert(::Type{ASerialization}, a::A) = ASerialization([a.x])
Base.convert(::Type{A}, a::ASerialization) = A(only(a.x))
If you do not want to overload Base.convert
then you can also define
JLD2.wconvert(::Type{ASerialization}, a::A) = ASerialization([a.x])
JLD2.rconvert(::Type{A}, a::ASerialization) = A(only(a.x))
instead. This may be particularly relevant when types are involved that are not your own.
struct B
x::Float64
end
JLD2.writeas(::Type{B}) = Float64
JLD2.wconvert(::Type{Float64}, b::B) = b.x
JLD2.rconvert(::Type{B}, x::Float64) = B(x)
arr = [B(rand()) for i=1:10]
jldsave("test.jld2"; arr)
In this example JLD2 converts the array of B
structs to a plain Vector{Float64}
prior to
storing to disk.