Foreword:

When I solved a plug-in problem today, I found that SO was not loaded normally, which was very strange. Finally, I found out that it was caused by the wrong parameter input.This leads to the title of this article. How should the four parameters in BaseDexClassLoader be passed in, and what are passed in?

There are a total of 4 parameters, in terms of points.

1: dexFile (String type)2: optimizedDirectory (File type)3: librarySearchPath (String type)4: parent (ClassLoader type)

One.dexPath(String)

Official note:

 * @param dexPath the list of jar/apk files containing classes and* resources, delimited by {@code File.pathSeparator}, which* defaults to {@code ":"} on Android.

List of jar/apk files containing classes and resources, separated by {@code File.pathSeparator}, where the default on Android is {@code":"}.

In other words, a collection can be passed in here.

For example, the following parameters are acceptable:

sdcard/test/aa.apk

sdcard/test/aa.apk:sdcard/test/bb.apk:sdcard/test/cc.apk

sdcard/test/aa.apk:sdcard/test/dd.jar

Where the separator:, to be on the safe side, you can use File.pathSeparator instead.

Sample code below:

private void loadDex(List apkList) {StringBuilder builder = new StringBuilder();for (File f : apkList) {builder.append(f.getAbsolutePath());builder.append(File.separatorChar);}DexClassLoader dexClassLoader = new DexClassLoader(builder.toString(), null, null, getClass().getClassLoader());}

two.optimizedDirectory

Official note:

this parameter is deprecated and has no effect since API level 26.

The path of decompression, the main purpose of passing the path here is to generate the odex folder, which is convenient for subsequent storage of odex files.

As written in the comments, this parameter 26 has initially been disabled.Therefore, it will not be expanded here.

Three.librarySearchPath

Official note:

 * @param librarySearchPath the list of directories containing native

Contains the directory list of the native directory. It should be noted here that the incoming directory must be the upper-level directory of so.If it is a higher-level directory, it will not work.The problem in the preface is here, a higher directory address is passed in.

Finding process:

The final place to use librarySearchPath is in the splitPaths method of DexPathList.Generate File and add it to List:

@UnsupportedAppUsageprivate static List splitPaths(String searchPath, boolean directoriesOnly) {List result = new ArrayList<>();if (searchPath != null) {for (String path : searchPath.split(File.pathSeparator)) {...result.add(new File(path));}}return result;}

When using it, the findLibrary method is used, and the for loop facilitates all the paths in the above collection to see if they exist.

public String findLibrary(String libraryName) {String fileName = System.mapLibraryName(libraryName);for (NativeLibraryElement element : nativeLibraryPathElements) {String path = element.findNativeLibrary(fileName);if (path != null) {return path;}}return null;}

The search will eventually call the findNativeLibrary method of NativeLibraryElement:

public String findNativeLibrary(String name) {maybeInit();if (zipDir == null) {String entryPath = new File(path, name).getPath();if (IoUtils.canOpenReadOnly(entryPath)) {return entryPath;}} else if (urlHandler != null) {// Having a urlHandler means the element has a zip file.// In this case Android supports loading the library iff// it is stored in the zip uncompressed.String entryName = zipDir + '/' + name;if (urlHandler.isEntryStored(entryName)) {return path.getPath() + zipSeparator + entryName;}}return null;}

This code is the core of the problem. It directly uses the method of new File(path,name) instead of looping search, so only the upper layer can be used.

four.parent

Official note:

@param parent the parent class loader

This is relatively simple, it is the classLoader of the previous layer.

V. Summary

Both dexPath and librarySearchPath support multi-path incoming.Use File.pathSeparator to separate paths.The dexPath must be the path of the APK or Jar package, and the librarySearchPath must be the upper-level folder of the so file.