JNI_OnLoad調用時機

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       JNI_OnLoad是在加載so的時候調用的，也就是System.loadLibrary("test")時候調用的。具體調用步驟如下：

System.loadLibrary(libName)->Runtime.loadLibrary(libName,classLoader)->Runtime.doLoad(libName,loader)->Runtime.nativeLoad(name,loader,ldLibraryPath)->JavaVMExt.loadNativieLibrary:(JavaVMExt* vm=Runtime::Current()->GetJavaVM(); vm->LoadNativieLibrary(filename,classloader,detail))->jni_internal.cc::LoadNativieLibrary->dlopen,dlsym->JNI_OnLoad。

以上給出了調用順序，具體代碼可以根據類名或方法名看出。有興趣的可以自己查看下源碼。

在Runtime.loadLibrary中，會在多個多個路徑下尋找對應的so，只要找到了就會去加載，加載成功就會返回。對應代碼如下：

Runtime.java

    /*
     * Searches for and loads the given shared library using the given ClassLoader.
     */
    void loadLibrary(String libraryName, ClassLoader loader) {
        if (loader != null) {
            String filename = loader.findLibrary(libraryName);
            if (filename == null) {
                // It's not necessarily true that the ClassLoader used
                // System.mapLibraryName, but the default setup does, and it's
                // misleading to say we didn't find "libMyLibrary.so" when we
                // actually searched for "liblibMyLibrary.so.so".
                throw new UnsatisfiedLinkError(loader + " couldn't find \"" +
                                               System.mapLibraryName(libraryName) + "\"");
            }
            String error = doLoad(filename, loader);
            if (error != null) {
                throw new UnsatisfiedLinkError(error);
            }
            return;
        }

        String filename = System.mapLibraryName(libraryName);
        List<String> candidates = new ArrayList<String>();
        String lastError = null;
        for (String directory : mLibPaths) {
            String candidate = directory + filename;
            candidates.add(candidate);

            if (IoUtils.canOpenReadOnly(candidate)) {
                String error = doLoad(candidate, loader);
                if (error == null) {
                    return; // We successfully loaded the library. Job done.
                }
                lastError = error;
            }
        }

        if (lastError != null) {
            throw new UnsatisfiedLinkError(lastError);
        }
        throw new UnsatisfiedLinkError("Library " + libraryName + " not found; tried " + candidates);
    }

mLibPaths初始化代碼如下：

    /**
     * Holds the library paths, used for native library lookup.
     */
    private final String[] mLibPaths = initLibPaths();

    private static String[] initLibPaths() {
        String javaLibraryPath = System.getProperty("java.library.path");
        if (javaLibraryPath == null) {
            return EmptyArray.STRING;
        }
        String[] paths = javaLibraryPath.split(":");
        // Add a '/' to the end of each directory so we don't have to do it every time.
        for (int i = 0; i < paths.length; ++i) {
            if (!paths[i].endsWith("/")) {
                paths[i] += "/";
            }
        }
        return paths;
    }