Tutorial :Prevent Java from loading library more than once



Question:

I have several classes that use the same JNI library, called "jni". In each of those classes, I have a System.loadLibrary() call in the class's static initializer:

Class class1 {      static{          System.loadLibrary("jni");      }      ...  }    Class class2 {      static{          System.loadLibrary("jni");      }      ...  }  

The only problem is, I only want System.loadLibrary() to be called once, since it can take a while. Is there an easy and clean way to prevent multiple loadings of the same library?


Solution:1

If the library has already be loaded any subsequent calls to load the same library will be ignored.

System.loadLibrary:

Loads the system library specified by the libname argument. The manner in which a library name is mapped to the actual system library is system dependent.

The call System.loadLibrary(name) is effectively equivalent to the call

 Runtime.getRuntime().loadLibrary(name)  

Runtime.loadLibrary:

Loads the dynamic library with the specified library name. A file containing native code is loaded from the local file system from a place where library files are conventionally obtained. The details of this process are implementation-dependent. The mapping from a library name to a specific filename is done in a system-specific manner.

First, if there is a security manager, its checkLink method is called with the libname as its argument. This may result in a security exception.

The method System.loadLibrary(String) is the conventional and convenient means of invoking this method. If native methods are to be used in the implementation of a class, a standard strategy is to put the native code in a library file (call it LibFile) and then to put a static initializer:

 static { System.loadLibrary("LibFile"); }  

within the class declaration. When the class is loaded and initialized, the necessary native code implementation for the native methods will then be loaded as well.

If this method is called more than once with the same library name, the second and subsequent calls are ignored.


Solution:2

From the JavaDocs -

If this method is called more than once with the same library name, the second and subsequent calls are ignored.


Solution:3

I'm pretty sure any subsequent calls to load an already loaded library have no effect [citation needed].

This was verified in the ClassLoader class source code, but I'm still trying to find a good source for this.


Solution:4

Create a singleton class which load "jni" only during it's first instantiation, and call this singleton in class1 and class2 instead of System.loadLibrary("jni")


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