Android ClassLoader源码分析

桃扇骨 2022-03-09 09:18 244阅读 0赞

转载请注明链接:[https://blog.csdn.net/feather\_wch/article/details/88428618][https_blog.csdn.net_feather_wch_article_details_88428618]

# Android ClassLoader源码分析 #

版本: 2019-03-12

--------------------

### 文章目录 ###

*  Android ClassLoader源码分析
 *   *  BaseDexClassLoader
     *   *  DexPathList

## BaseDexClassLoader ##

1、BaseDexClassLoader内部重要概念总结

> 1.  `DexPathList pathList`: 内部有一个dexElements数组，存储着该ClassLoader的所有dex/resources的路径

1、BaseDexClassLoader源码

package dalvik.system;
    
    /** * Base class for common functionality between various dex-based * {@link ClassLoader} implementations. */
    public class BaseDexClassLoader extends ClassLoader { 
    
        /** * Hook for customizing how dex files loads are reported. * * This enables the framework to monitor the use of dex files. The * goal is to simplify the mechanism for optimizing foreign dex files and * enable further optimizations of secondary dex files. * * The reporting happens only when new instances of BaseDexClassLoader * are constructed and will be active only after this field is set with * {@link BaseDexClassLoader#setReporter}. */
        /* @NonNull */ private static volatile Reporter reporter = null;
    
        private final DexPathList pathList;
    
        /** * Constructs an instance. * Note that all the *.jar and *.apk files from {@code dexPath} might be * first extracted in-memory before the code is loaded. This can be avoided * by passing raw dex files (*.dex) in the {@code dexPath}. * * @param dexPath the list of jar/apk files containing classes and * resources, delimited by {@code File.pathSeparator}, which * defaults to {@code ":"} on Android. * @param optimizedDirectory this parameter is deprecated and has no effect since API level 26. * @param librarySearchPath the list of directories containing native * libraries, delimited by {@code File.pathSeparator}; may be * {@code null} * @param parent the parent class loader */
        public BaseDexClassLoader(String dexPath, File optimizedDirectory,
                String librarySearchPath, ClassLoader parent) { 
            this(dexPath, optimizedDirectory, librarySearchPath, parent, false);
        }
    
        /** * @hide */
        public BaseDexClassLoader(String dexPath, File optimizedDirectory,
                String librarySearchPath, ClassLoader parent, boolean isTrusted) { 
            super(parent);
            this.pathList = new DexPathList(this, dexPath, librarySearchPath, null, isTrusted);
    
            if (reporter != null) { 
                reportClassLoaderChain();
            }
        }
    
        /** * Reports the current class loader chain to the registered {@code reporter}. * The chain is reported only if all its elements are {@code BaseDexClassLoader}. */
        private void reportClassLoaderChain() { 
            ArrayList<BaseDexClassLoader> classLoadersChain = new ArrayList<>();
            ArrayList<String> classPaths = new ArrayList<>();
    
            classLoadersChain.add(this);
            classPaths.add(String.join(File.pathSeparator, pathList.getDexPaths()));
    
            boolean onlySawSupportedClassLoaders = true;
            ClassLoader bootClassLoader = ClassLoader.getSystemClassLoader().getParent();
            ClassLoader current = getParent();
    
            while (current != null && current != bootClassLoader) { 
                if (current instanceof BaseDexClassLoader) { 
                    BaseDexClassLoader bdcCurrent = (BaseDexClassLoader) current;
                    classLoadersChain.add(bdcCurrent);
                    classPaths.add(String.join(File.pathSeparator, bdcCurrent.pathList.getDexPaths()));
                } else { 
                    onlySawSupportedClassLoaders = false;
                    break;
                }
                current = current.getParent();
            }
    
            if (onlySawSupportedClassLoaders) { 
                reporter.report(classLoadersChain, classPaths);
            }
        }
    
        /** * Constructs an instance. * * dexFile must be an in-memory representation of a full dexFile. * * @param dexFiles the array of in-memory dex files containing classes. * @param parent the parent class loader * * @hide */
        public BaseDexClassLoader(ByteBuffer[] dexFiles, ClassLoader parent) { 
            // TODO We should support giving this a library search path maybe.
            super(parent);
            this.pathList = new DexPathList(this, dexFiles);
        }
    
        @Override
        protected Class<?> findClass(String name) throws ClassNotFoundException { 
            List<Throwable> suppressedExceptions = new ArrayList<Throwable>();
            Class c = pathList.findClass(name, suppressedExceptions);
            if (c == null) { 
                ClassNotFoundException cnfe = new ClassNotFoundException(
                        "Didn't find class \"" + name + "\" on path: " + pathList);
                for (Throwable t : suppressedExceptions) { 
                    cnfe.addSuppressed(t);
                }
                throw cnfe;
            }
            return c;
        }
    
        /** * @hide */
        public void addDexPath(String dexPath) { 
            addDexPath(dexPath, false /*isTrusted*/);
        }
    
        /** * @hide */
        public void addDexPath(String dexPath, boolean isTrusted) { 
            pathList.addDexPath(dexPath, null /*optimizedDirectory*/, isTrusted);
        }
    
        /** * Adds additional native paths for consideration in subsequent calls to * {@link #findLibrary(String)} * @hide */
        public void addNativePath(Collection<String> libPaths) { 
            pathList.addNativePath(libPaths);
        }
    
        @Override
        protected URL findResource(String name) { 
            return pathList.findResource(name);
        }
    
        @Override
        protected Enumeration<URL> findResources(String name) { 
            return pathList.findResources(name);
        }
    
        @Override
        public String findLibrary(String name) { 
            return pathList.findLibrary(name);
        }
    
        /** * Returns package information for the given package. * Unfortunately, instances of this class don't really have this * information, and as a non-secure {@code ClassLoader}, it isn't * even required to, according to the spec. Yet, we want to * provide it, in order to make all those hopeful callers of * {@code myClass.getPackage().getName()} happy. Thus we construct * a {@code Package} object the first time it is being requested * and fill most of the fields with dummy values. The {@code * Package} object is then put into the {@code ClassLoader}'s * package cache, so we see the same one next time. We don't * create {@code Package} objects for {@code null} arguments or * for the default package. * * <p>There is a limited chance that we end up with multiple * {@code Package} objects representing the same package: It can * happen when when a package is scattered across different JAR * files which were loaded by different {@code ClassLoader} * instances. This is rather unlikely, and given that this whole * thing is more or less a workaround, probably not worth the * effort to address. * * @param name the name of the class * @return the package information for the class, or {@code null} * if there is no package information available for it */
        @Override
        protected synchronized Package getPackage(String name) { 
            if (name != null && !name.isEmpty()) { 
                Package pack = super.getPackage(name);
    
                if (pack == null) { 
                    pack = definePackage(name, "Unknown", "0.0", "Unknown",
                            "Unknown", "0.0", "Unknown", null);
                }
    
                return pack;
            }
    
            return null;
        }
    
        /** * @hide */
        public String getLdLibraryPath() { 
            StringBuilder result = new StringBuilder();
            for (File directory : pathList.getNativeLibraryDirectories()) { 
                if (result.length() > 0) { 
                    result.append(':');
                }
                result.append(directory);
            }
    
            return result.toString();
        }
    
        @Override public String toString() { 
            return getClass().getName() + "[" + pathList + "]";
        }
    
        /** * Sets the reporter for dex load notifications. * Once set, all new instances of BaseDexClassLoader will report upon * constructions the loaded dex files. * * @param newReporter the new Reporter. Setting null will cancel reporting. * @hide */
        public static void setReporter(Reporter newReporter) { 
            reporter = newReporter;
        }
    
        /** * @hide */
        public static Reporter getReporter() { 
            return reporter;
        }
    
        /** * @hide */
        public interface Reporter { 
            /** * Reports the construction of a BaseDexClassLoader and provides information about the * class loader chain. * Note that this only reports if all class loader in the chain are BaseDexClassLoader. * * @param classLoadersChain the chain of class loaders used during the construction of the * class loader. The first element is the BaseDexClassLoader being constructed, * the second element is its parent, and so on. * @param classPaths the class paths of the class loaders present in * {@param classLoadersChain}. The first element corresponds to the first class * loader and so on. A classpath is represented as a list of dex files separated by * {@code File.pathSeparator}. */
            void report(List<BaseDexClassLoader> classLoadersChain, List<String> classPaths);
        }
    }

### DexPathList ###

1、DexPathList源码

package dalvik.system;
    
    /** * A pair of lists of entries, associated with a {@code ClassLoader}. * One of the lists is a dex/resource path — typically referred * to as a "class path" — list, and the other names directories * containing native code libraries. Class path entries may be any of: * a {@code .jar} or {@code .zip} file containing an optional * top-level {@code classes.dex} file as well as arbitrary resources, * or a plain {@code .dex} file (with no possibility of associated * resources). * * <p>This class also contains methods to use these lists to look up * classes and resources.</p> */
    /*package*/ final class DexPathList { 
        private static final String DEX_SUFFIX = ".dex";
        private static final String zipSeparator = "!/";
    
        /** class definition context */
        private final ClassLoader definingContext;
    
        /** * List of dex/resource (class path) elements. * Should be called pathElements, but the Facebook app uses reflection * to modify 'dexElements' (http://b/7726934). */
        private Element[] dexElements;
    
        /** List of native library path elements. */
        // Some applications rely on this field being an array or we'd use a final list here
        /* package visible for testing */ NativeLibraryElement[] nativeLibraryPathElements;
    
        /** List of application native library directories. */
        private final List<File> nativeLibraryDirectories;
    
        /** List of system native library directories. */
        private final List<File> systemNativeLibraryDirectories;
    
        /** * Exceptions thrown during creation of the dexElements list. */
        private IOException[] dexElementsSuppressedExceptions;
    
        /** * Construct an instance. * * @param definingContext the context in which any as-yet unresolved * classes should be defined * * @param dexFiles the bytebuffers containing the dex files that we should load classes from. */
        public DexPathList(ClassLoader definingContext, ByteBuffer[] dexFiles) { 
            if (definingContext == null) { 
                throw new NullPointerException("definingContext == null");
            }
            if (dexFiles == null) { 
                throw new NullPointerException("dexFiles == null");
            }
            if (Arrays.stream(dexFiles).anyMatch(v -> v == null)) { 
                throw new NullPointerException("dexFiles contains a null Buffer!");
            }
    
            this.definingContext = definingContext;
            // TODO It might be useful to let in-memory dex-paths have native libraries.
            this.nativeLibraryDirectories = Collections.emptyList();
            this.systemNativeLibraryDirectories =
                    splitPaths(System.getProperty("java.library.path"), true);
            this.nativeLibraryPathElements = makePathElements(this.systemNativeLibraryDirectories);
    
            ArrayList<IOException> suppressedExceptions = new ArrayList<IOException>();
            this.dexElements = makeInMemoryDexElements(dexFiles, suppressedExceptions);
            if (suppressedExceptions.size() > 0) { 
                this.dexElementsSuppressedExceptions =
                        suppressedExceptions.toArray(new IOException[suppressedExceptions.size()]);
            } else { 
                dexElementsSuppressedExceptions = null;
            }
        }
    
        /** * Constructs an instance. * * @param definingContext the context in which any as-yet unresolved * classes should be defined * @param dexPath list of dex/resource path elements, separated by * {@code File.pathSeparator} * @param librarySearchPath list of native library directory path elements, * separated by {@code File.pathSeparator} * @param optimizedDirectory directory where optimized {@code .dex} files * should be found and written to, or {@code null} to use the default * system directory for same */
        public DexPathList(ClassLoader definingContext, String dexPath,
                String librarySearchPath, File optimizedDirectory) { 
            this(definingContext, dexPath, librarySearchPath, optimizedDirectory, false);
        }
    
        DexPathList(ClassLoader definingContext, String dexPath,
                String librarySearchPath, File optimizedDirectory, boolean isTrusted) { 
            if (definingContext == null) { 
                throw new NullPointerException("definingContext == null");
            }
    
            if (dexPath == null) { 
                throw new NullPointerException("dexPath == null");
            }
    
            if (optimizedDirectory != null) { 
                if (!optimizedDirectory.exists())  { 
                    throw new IllegalArgumentException(
                            "optimizedDirectory doesn't exist: "
                            + optimizedDirectory);
                }
    
                if (!(optimizedDirectory.canRead()
                                && optimizedDirectory.canWrite())) { 
                    throw new IllegalArgumentException(
                            "optimizedDirectory not readable/writable: "
                            + optimizedDirectory);
                }
            }
    
            this.definingContext = definingContext;
    
            ArrayList<IOException> suppressedExceptions = new ArrayList<IOException>();
            // save dexPath for BaseDexClassLoader
            this.dexElements = makeDexElements(splitDexPath(dexPath), optimizedDirectory,
                                               suppressedExceptions, definingContext, isTrusted);
    
            // Native libraries may exist in both the system and
            // application library paths, and we use this search order:
            //
            // 1. This class loader's library path for application libraries (librarySearchPath):
            // 1.1. Native library directories
            // 1.2. Path to libraries in apk-files
            // 2. The VM's library path from the system property for system libraries
            // also known as java.library.path
            //
            // This order was reversed prior to Gingerbread; see http://b/2933456.
            this.nativeLibraryDirectories = splitPaths(librarySearchPath, false);
            this.systemNativeLibraryDirectories =
                    splitPaths(System.getProperty("java.library.path"), true);
            List<File> allNativeLibraryDirectories = new ArrayList<>(nativeLibraryDirectories);
            allNativeLibraryDirectories.addAll(systemNativeLibraryDirectories);
    
            this.nativeLibraryPathElements = makePathElements(allNativeLibraryDirectories);
    
            if (suppressedExceptions.size() > 0) { 
                this.dexElementsSuppressedExceptions =
                    suppressedExceptions.toArray(new IOException[suppressedExceptions.size()]);
            } else { 
                dexElementsSuppressedExceptions = null;
            }
        }
    
        @Override public String toString() { 
            List<File> allNativeLibraryDirectories = new ArrayList<>(nativeLibraryDirectories);
            allNativeLibraryDirectories.addAll(systemNativeLibraryDirectories);
    
            File[] nativeLibraryDirectoriesArray =
                    allNativeLibraryDirectories.toArray(
                        new File[allNativeLibraryDirectories.size()]);
    
            return "DexPathList[" + Arrays.toString(dexElements) +
                ",nativeLibraryDirectories=" + Arrays.toString(nativeLibraryDirectoriesArray) + "]";
        }
    
        /** * For BaseDexClassLoader.getLdLibraryPath. */
        public List<File> getNativeLibraryDirectories() { 
            return nativeLibraryDirectories;
        }
    
        /** * Adds a new path to this instance * @param dexPath list of dex/resource path element, separated by * {@code File.pathSeparator} * @param optimizedDirectory directory where optimized {@code .dex} files * should be found and written to, or {@code null} to use the default * system directory for same */
        public void addDexPath(String dexPath, File optimizedDirectory) { 
          addDexPath(dexPath, optimizedDirectory, false);
        }
    
        public void addDexPath(String dexPath, File optimizedDirectory, boolean isTrusted) { 
            final List<IOException> suppressedExceptionList = new ArrayList<IOException>();
            final Element[] newElements = makeDexElements(splitDexPath(dexPath), optimizedDirectory,
                    suppressedExceptionList, definingContext, isTrusted);
    
            if (newElements != null && newElements.length > 0) { 
                final Element[] oldElements = dexElements;
                dexElements = new Element[oldElements.length + newElements.length];
                System.arraycopy(
                        oldElements, 0, dexElements, 0, oldElements.length);
                System.arraycopy(
                        newElements, 0, dexElements, oldElements.length, newElements.length);
            }
    
            if (suppressedExceptionList.size() > 0) { 
                final IOException[] newSuppressedExceptions = suppressedExceptionList.toArray(
                        new IOException[suppressedExceptionList.size()]);
                if (dexElementsSuppressedExceptions != null) { 
                    final IOException[] oldSuppressedExceptions = dexElementsSuppressedExceptions;
                    final int suppressedExceptionsLength = oldSuppressedExceptions.length +
                            newSuppressedExceptions.length;
                    dexElementsSuppressedExceptions = new IOException[suppressedExceptionsLength];
                    System.arraycopy(oldSuppressedExceptions, 0, dexElementsSuppressedExceptions,
                            0, oldSuppressedExceptions.length);
                    System.arraycopy(newSuppressedExceptions, 0, dexElementsSuppressedExceptions,
                            oldSuppressedExceptions.length, newSuppressedExceptions.length);
                } else { 
                    dexElementsSuppressedExceptions = newSuppressedExceptions;
                }
            }
        }
    
        /** * Splits the given dex path string into elements using the path * separator, pruning out any elements that do not refer to existing * and readable files. */
        private static List<File> splitDexPath(String path) { 
            return splitPaths(path, false);
        }
    
        /** * Splits the given path strings into file elements using the path * separator, combining the results and filtering out elements * that don't exist, aren't readable, or aren't either a regular * file or a directory (as specified). Either string may be empty * or {@code null}, in which case it is ignored. If both strings * are empty or {@code null}, or all elements get pruned out, then * this returns a zero-element list. */
        private static List<File> splitPaths(String searchPath, boolean directoriesOnly) { 
            List<File> result = new ArrayList<>();
    
            if (searchPath != null) { 
                for (String path : searchPath.split(File.pathSeparator)) { 
                    if (directoriesOnly) { 
                        try { 
                            StructStat sb = Libcore.os.stat(path);
                            if (!S_ISDIR(sb.st_mode)) { 
                                continue;
                            }
                        } catch (ErrnoException ignored) { 
                            continue;
                        }
                    }
                    result.add(new File(path));
                }
            }
    
            return result;
        }
    
        private static Element[] makeInMemoryDexElements(ByteBuffer[] dexFiles,
                List<IOException> suppressedExceptions) { 
            Element[] elements = new Element[dexFiles.length];
            int elementPos = 0;
            for (ByteBuffer buf : dexFiles) { 
                try { 
                    DexFile dex = new DexFile(buf);
                    elements[elementPos++] = new Element(dex);
                } catch (IOException suppressed) { 
                    System.logE("Unable to load dex file: " + buf, suppressed);
                    suppressedExceptions.add(suppressed);
                }
            }
            if (elementPos != elements.length) { 
                elements = Arrays.copyOf(elements, elementPos);
            }
            return elements;
        }
    
        /** * Makes an array of dex/resource path elements, one per element of * the given array. */
        private static Element[] makeDexElements(List<File> files, File optimizedDirectory,
                List<IOException> suppressedExceptions, ClassLoader loader) { 
            return makeDexElements(files, optimizedDirectory, suppressedExceptions, loader, false);
        }
    
    
        private static Element[] makeDexElements(List<File> files, File optimizedDirectory,
                List<IOException> suppressedExceptions, ClassLoader loader, boolean isTrusted) { 
          Element[] elements = new Element[files.size()];
          int elementsPos = 0;
          /* * Open all files and load the (direct or contained) dex files up front. */
          for (File file : files) { 
              if (file.isDirectory()) { 
                  // We support directories for looking up resources. Looking up resources in
                  // directories is useful for running libcore tests.
                  elements[elementsPos++] = new Element(file);
              } else if (file.isFile()) { 
                  String name = file.getName();
    
                  DexFile dex = null;
                  if (name.endsWith(DEX_SUFFIX)) { 
                      // Raw dex file (not inside a zip/jar).
                      try { 
                          dex = loadDexFile(file, optimizedDirectory, loader, elements);
                          if (dex != null) { 
                              elements[elementsPos++] = new Element(dex, null);
                          }
                      } catch (IOException suppressed) { 
                          System.logE("Unable to load dex file: " + file, suppressed);
                          suppressedExceptions.add(suppressed);
                      }
                  } else { 
                      try { 
                          dex = loadDexFile(file, optimizedDirectory, loader, elements);
                      } catch (IOException suppressed) { 
                          /* * IOException might get thrown "legitimately" by the DexFile constructor if * the zip file turns out to be resource-only (that is, no classes.dex file * in it). * Let dex == null and hang on to the exception to add to the tea-leaves for * when findClass returns null. */
                          suppressedExceptions.add(suppressed);
                      }
    
                      if (dex == null) { 
                          elements[elementsPos++] = new Element(file);
                      } else { 
                          elements[elementsPos++] = new Element(dex, file);
                      }
                  }
                  if (dex != null && isTrusted) { 
                    dex.setTrusted();
                  }
              } else { 
                  System.logW("ClassLoader referenced unknown path: " + file);
              }
          }
          if (elementsPos != elements.length) { 
              elements = Arrays.copyOf(elements, elementsPos);
          }
          return elements;
        }
    
        /** * Constructs a {@code DexFile} instance, as appropriate depending on whether * {@code optimizedDirectory} is {@code null}. An application image file may be associated with * the {@code loader} if it is not null. */
        private static DexFile loadDexFile(File file, File optimizedDirectory, ClassLoader loader,
                                           Element[] elements)
                throws IOException { 
            if (optimizedDirectory == null) { 
                return new DexFile(file, loader, elements);
            } else { 
                String optimizedPath = optimizedPathFor(file, optimizedDirectory);
                return DexFile.loadDex(file.getPath(), optimizedPath, 0, loader, elements);
            }
        }
    
        /** * Converts a dex/jar file path and an output directory to an * output file path for an associated optimized dex file. */
        private static String optimizedPathFor(File path,
                File optimizedDirectory) { 
            /* * Get the filename component of the path, and replace the * suffix with ".dex" if that's not already the suffix. * * We don't want to use ".odex", because the build system uses * that for files that are paired with resource-only jar * files. If the VM can assume that there's no classes.dex in * the matching jar, it doesn't need to open the jar to check * for updated dependencies, providing a slight performance * boost at startup. The use of ".dex" here matches the use on * files in /data/dalvik-cache. */
            String fileName = path.getName();
            if (!fileName.endsWith(DEX_SUFFIX)) { 
                int lastDot = fileName.lastIndexOf(".");
                if (lastDot < 0) { 
                    fileName += DEX_SUFFIX;
                } else { 
                    StringBuilder sb = new StringBuilder(lastDot + 4);
                    sb.append(fileName, 0, lastDot);
                    sb.append(DEX_SUFFIX);
                    fileName = sb.toString();
                }
            }
    
            File result = new File(optimizedDirectory, fileName);
            return result.getPath();
        }
    
        /* * TODO (dimitry): Revert after apps stops relying on the existence of this * method (see http://b/21957414 and http://b/26317852 for details) */
        @SuppressWarnings("unused")
        private static Element[] makePathElements(List<File> files, File optimizedDirectory,
                List<IOException> suppressedExceptions) { 
            return makeDexElements(files, optimizedDirectory, suppressedExceptions, null);
        }
    
        /** * Makes an array of directory/zip path elements for the native library search path, one per * element of the given array. */
        private static NativeLibraryElement[] makePathElements(List<File> files) { 
            NativeLibraryElement[] elements = new NativeLibraryElement[files.size()];
            int elementsPos = 0;
            for (File file : files) { 
                String path = file.getPath();
    
                if (path.contains(zipSeparator)) { 
                    String split[] = path.split(zipSeparator, 2);
                    File zip = new File(split[0]);
                    String dir = split[1];
                    elements[elementsPos++] = new NativeLibraryElement(zip, dir);
                } else if (file.isDirectory()) { 
                    // We support directories for looking up native libraries.
                    elements[elementsPos++] = new NativeLibraryElement(file);
                }
            }
            if (elementsPos != elements.length) { 
                elements = Arrays.copyOf(elements, elementsPos);
            }
            return elements;
        }
    
        /** * Finds the named class in one of the dex files pointed at by * this instance. This will find the one in the earliest listed * path element. If the class is found but has not yet been * defined, then this method will define it in the defining * context that this instance was constructed with. * * @param name of class to find * @param suppressed exceptions encountered whilst finding the class * @return the named class or {@code null} if the class is not * found in any of the dex files */
        public Class<?> findClass(String name, List<Throwable> suppressed) { 
            for (Element element : dexElements) { 
                Class<?> clazz = element.findClass(name, definingContext, suppressed);
                if (clazz != null) { 
                    return clazz;
                }
            }
    
            if (dexElementsSuppressedExceptions != null) { 
                suppressed.addAll(Arrays.asList(dexElementsSuppressedExceptions));
            }
            return null;
        }
    
        /** * Finds the named resource in one of the zip/jar files pointed at * by this instance. This will find the one in the earliest listed * path element. * * @return a URL to the named resource or {@code null} if the * resource is not found in any of the zip/jar files */
        public URL findResource(String name) { 
            for (Element element : dexElements) { 
                URL url = element.findResource(name);
                if (url != null) { 
                    return url;
                }
            }
    
            return null;
        }
    
        /** * Finds all the resources with the given name, returning an * enumeration of them. If there are no resources with the given * name, then this method returns an empty enumeration. */
        public Enumeration<URL> findResources(String name) { 
            ArrayList<URL> result = new ArrayList<URL>();
    
            for (Element element : dexElements) { 
                URL url = element.findResource(name);
                if (url != null) { 
                    result.add(url);
                }
            }
    
            return Collections.enumeration(result);
        }
    
        /** * Finds the named native code library on any of the library * directories pointed at by this instance. This will find the * one in the earliest listed directory, ignoring any that are not * readable regular files. * * @return the complete path to the library or {@code null} if no * library was found */
        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;
        }
    
        /** * Returns the list of all individual dex files paths from the current list. * The list will contain only file paths (i.e. no directories). */
        /*package*/ List<String> getDexPaths() { 
            List<String> dexPaths = new ArrayList<String>();
            for (Element e : dexElements) { 
                String dexPath = e.getDexPath();
                if (dexPath != null) { 
                    // Add the element to the list only if it is a file. A null dex path signals the
                    // element is a resource directory or an in-memory dex file.
                    dexPaths.add(dexPath);
                }
            }
            return dexPaths;
        }
    
        /** * Adds a collection of library paths from which to load native libraries. Paths can be absolute * native library directories (i.e. /data/app/foo/lib/arm64) or apk references (i.e. * /data/app/foo/base.apk!/lib/arm64). * * Note: This method will attempt to dedupe elements. * Note: This method replaces the value of {@link #nativeLibraryPathElements} */
        public void addNativePath(Collection<String> libPaths) { 
            if (libPaths.isEmpty()) { 
                return;
            }
            List<File> libFiles = new ArrayList<>(libPaths.size());
            for (String path : libPaths) { 
                libFiles.add(new File(path));
            }
            ArrayList<NativeLibraryElement> newPaths =
                    new ArrayList<>(nativeLibraryPathElements.length + libPaths.size());
            newPaths.addAll(Arrays.asList(nativeLibraryPathElements));
            for (NativeLibraryElement element : makePathElements(libFiles)) { 
                if (!newPaths.contains(element)) { 
                    newPaths.add(element);
                }
            }
            nativeLibraryPathElements = newPaths.toArray(new NativeLibraryElement[newPaths.size()]);
        }
    
        /** * Element of the dex/resource path. Note: should be called DexElement, but apps reflect on * this. */
        /*package*/ static class Element { 
            /** * A file denoting a zip file (in case of a resource jar or a dex jar), or a directory * (only when dexFile is null). */
            private final File path;
    
            private final DexFile dexFile;
    
            private ClassPathURLStreamHandler urlHandler;
            private boolean initialized;
    
            /** * Element encapsulates a dex file. This may be a plain dex file (in which case dexZipPath * should be null), or a jar (in which case dexZipPath should denote the zip file). */
            public Element(DexFile dexFile, File dexZipPath) { 
                this.dexFile = dexFile;
                this.path = dexZipPath;
            }
    
            public Element(DexFile dexFile) { 
                this.dexFile = dexFile;
                this.path = null;
            }
    
            public Element(File path) { 
              this.path = path;
              this.dexFile = null;
            }
    
            /** * Constructor for a bit of backwards compatibility. Some apps use reflection into * internal APIs. Warn, and emulate old behavior if we can. See b/33399341. * * @deprecated The Element class has been split. Use new Element constructors for * classes and resources, and NativeLibraryElement for the library * search path. */
            @Deprecated
            public Element(File dir, boolean isDirectory, File zip, DexFile dexFile) { 
                System.err.println("Warning: Using deprecated Element constructor. Do not use internal"
                        + " APIs, this constructor will be removed in the future.");
                if (dir != null && (zip != null || dexFile != null)) { 
                    throw new IllegalArgumentException("Using dir and zip|dexFile no longer"
                            + " supported.");
                }
                if (isDirectory && (zip != null || dexFile != null)) { 
                    throw new IllegalArgumentException("Unsupported argument combination.");
                }
                if (dir != null) { 
                    this.path = dir;
                    this.dexFile = null;
                } else { 
                    this.path = zip;
                    this.dexFile = dexFile;
                }
            }
    
            /* * Returns the dex path of this element or null if the element refers to a directory. */
            private String getDexPath() { 
                if (path != null) { 
                    return path.isDirectory() ? null : path.getAbsolutePath();
                } else if (dexFile != null) { 
                    // DexFile.getName() returns the path of the dex file.
                    return dexFile.getName();
                }
                return null;
            }
    
            @Override
            public String toString() { 
                if (dexFile == null) { 
                  return (path.isDirectory() ? "directory \"" : "zip file \"") + path + "\"";
                } else { 
                  if (path == null) { 
                    return "dex file \"" + dexFile + "\"";
                  } else { 
                    return "zip file \"" + path + "\"";
                  }
                }
            }
    
            public synchronized void maybeInit() { 
                if (initialized) { 
                    return;
                }
    
                if (path == null || path.isDirectory()) { 
                    initialized = true;
                    return;
                }
    
                try { 
                    urlHandler = new ClassPathURLStreamHandler(path.getPath());
                } catch (IOException ioe) { 
                    /* * Note: ZipException (a subclass of IOException) * might get thrown by the ZipFile constructor * (e.g. if the file isn't actually a zip/jar * file). */
                    System.logE("Unable to open zip file: " + path, ioe);
                    urlHandler = null;
                }
    
                // Mark this element as initialized only after we've successfully created
                // the associated ClassPathURLStreamHandler. That way, we won't leave this
                // element in an inconsistent state if an exception is thrown during initialization.
                //
                // See b/35633614.
                initialized = true;
            }
    
            public Class<?> findClass(String name, ClassLoader definingContext,
                    List<Throwable> suppressed) { 
                return dexFile != null ? dexFile.loadClassBinaryName(name, definingContext, suppressed)
                        : null;
            }
    
            public URL findResource(String name) { 
                maybeInit();
    
                if (urlHandler != null) { 
                  return urlHandler.getEntryUrlOrNull(name);
                }
    
                // We support directories so we can run tests and/or legacy code
                // that uses Class.getResource.
                if (path != null && path.isDirectory()) { 
                    File resourceFile = new File(path, name);
                    if (resourceFile.exists()) { 
                        try { 
                            return resourceFile.toURI().toURL();
                        } catch (MalformedURLException ex) { 
                            throw new RuntimeException(ex);
                        }
                    }
                }
    
                return null;
            }
        }
    
        /** * Element of the native library path */
        /*package*/ static class NativeLibraryElement { 
            /** * A file denoting a directory or zip file. */
            private final File path;
    
            /** * If path denotes a zip file, this denotes a base path inside the zip. */
            private final String zipDir;
    
            private ClassPathURLStreamHandler urlHandler;
            private boolean initialized;
    
            public NativeLibraryElement(File dir) { 
                this.path = dir;
                this.zipDir = null;
    
                // We should check whether path is a directory, but that is non-eliminatable overhead.
            }
    
            public NativeLibraryElement(File zip, String zipDir) { 
                this.path = zip;
                this.zipDir = zipDir;
    
                // Simple check that should be able to be eliminated by inlining. We should also
                // check whether path is a file, but that is non-eliminatable overhead.
                if (zipDir == null) { 
                  throw new IllegalArgumentException();
                }
            }
    
            @Override
            public String toString() { 
                if (zipDir == null) { 
                    return "directory \"" + path + "\"";
                } else { 
                    return "zip file \"" + path + "\"" +
                      (!zipDir.isEmpty() ? ", dir \"" + zipDir + "\"" : "");
                }
            }
    
            public synchronized void maybeInit() { 
                if (initialized) { 
                    return;
                }
    
                if (zipDir == null) { 
                    initialized = true;
                    return;
                }
    
                try { 
                    urlHandler = new ClassPathURLStreamHandler(path.getPath());
                } catch (IOException ioe) { 
                    /* * Note: ZipException (a subclass of IOException) * might get thrown by the ZipFile constructor * (e.g. if the file isn't actually a zip/jar * file). */
                    System.logE("Unable to open zip file: " + path, ioe);
                    urlHandler = null;
                }
    
                // Mark this element as initialized only after we've successfully created
                // the associated ClassPathURLStreamHandler. That way, we won't leave this
                // element in an inconsistent state if an exception is thrown during initialization.
                //
                // See b/35633614.
                initialized = true;
            }
    
            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;
            }
    
            @Override
            public boolean equals(Object o) { 
                if (this == o) return true;
                if (!(o instanceof NativeLibraryElement)) return false;
                NativeLibraryElement that = (NativeLibraryElement) o;
                return Objects.equals(path, that.path) &&
                        Objects.equals(zipDir, that.zipDir);
            }
    
            @Override
            public int hashCode() { 
                return Objects.hash(path, zipDir);
            }
        }
    }

[https_blog.csdn.net_feather_wch_article_details_88428618]: https://blog.csdn.net/feather_wch/article/details/88428618