Android類加載器源碼分析

基礎概念

• Android中的ClassLoader類型可分爲系統ClassLoader和自定義ClassLoader。其中系統ClassLoader包括3種分別是：

  • BootClassLoader，Android系統啓動時會使用BootClassLoader來預加載經常使用類，與Java中的Bootstrap ClassLoader不一樣的是，它並非由C/C++代碼實現，而是由Java實現的。BootClassLoader是ClassLoader的一個內部類。

    • PathClassLoader，只能加載系統中已經安裝過的apk。
    • DexClassLoader，是一個能夠從包含classes.dex實體的.jar或.apk文件中加載classes的類加載器。能夠用於實現dex的動態加載、代碼熱更新等等。能夠加載jar/apk/dex，能夠從SD卡中加載未安裝的apk。
    • PathClassLoader和DexClasLoader都是繼承自 BaseDexClassLoader，它們的類加載邏輯所有寫在BaseDexClassLoader中。

源碼分析

DexClassLoader分析

DexClassLoader.java

位置：~/android6.0.1_r66/libcore/dalvik/src/main/java/dalvik/system/DexClassLoader.java
繼承BaseDexClassLoader

public class DexClassLoader extends BaseDexClassLoader {   
    public DexClassLoader(String dexPath, String optimizedDirectory,
            String libraryPath, ClassLoader parent) {
        super(dexPath, new File(optimizedDirectory), libraryPath, parent);
    }
}

參數：

• dexpath：jar或apk文件目錄
• optimizedDirectory：優化dex緩存目錄
• libraryPath：包含native lib的目錄路徑
• parent：父類加載器

BaseDexClassLoader.java

位置：~/android6.0.1_r66/libcore/dalvik/src/main/java/dalvik/system/BaseDexClassLoader.java
繼承ClassLoader

public class BaseDexClassLoader extends ClassLoader {
    private final DexPathList pathList;
     public BaseDexClassLoader(String dexPath, File optimizedDirectory,
            String libraryPath, ClassLoader parent) {
        super(parent);
        this.pathList = new DexPathList(this, dexPath, libraryPath, optimizedDirectory);
    }
    ...
}

首先調用父類的構造函數super(parent)：ClassLoader
而後調用並初始化了DexPathList類型的對象pathList。

ClassLoader.java

位置：~/android6.0.1_r66/libcore/libart/src/main/java/java/lang/ClassLoader.java

public abstract class ClassLoader {
...
protected ClassLoader(ClassLoader parentLoader) {
        this(parentLoader, false);
    }
ClassLoader(ClassLoader parentLoader, boolean nullAllowed) {
        if (parentLoader == null && !nullAllowed) {
            throw new NullPointerException("parentLoader == null && !nullAllowed");
        }
        parent = parentLoader;
    }
...
}

該構造函數把傳進來的父類加載器賦給了私有變量parent。

DexPathList.java

位置：~/android6.0.1_r66/libcore/dalvik/src/main/java/dalvik/system/DexPathList.java

final class DexPathList {
    private static final String DEX_SUFFIX = ".dex";
    private static final String zipSeparator = "!/";
    
    ...
    
    public DexPathList(ClassLoader definingContext, String dexPath,
            String libraryPath, File optimizedDirectory) {

        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 = makePathElements(splitDexPath(dexPath), optimizedDirectory,                                            suppressedExceptions);
                                             this.nativeLibraryDirectories = splitPaths(libraryPath, false);
        this.systemNativeLibraryDirectories =                splitPaths(System.getProperty("java.library.path"), true);
        List<File> allNativeLibraryDirectories = new ArrayList<>(nativeLibraryDirectories);
                allNativeLibraryDirectories.addAll(systemNativeLibraryDirectories);

        this.nativeLibraryPathElements = makePathElements(allNativeLibraryDirectories, null,
                                                          suppressedExceptions);

        if (suppressedExceptions.size() > 0) {
            this.dexElementsSuppressedExceptions =
                suppressedExceptions.toArray(new IOException[suppressedExceptions.size()]);
        } else {
            dexElementsSuppressedExceptions = null;
        }
    }
}

首先對傳入參數的驗證，而後調用makeDexElements(List<File> files, File optimizedDirectory, List<IOException> suppressedExceptions, ClassLoader loader)方法

private static Element[] makePathElements(List<File> files, File optimizedDirectory,
                                              List<IOException> suppressedExceptions) {
        List<Element> elements = new ArrayList<>();
        /*
         * Open all files and load the (direct or contained) dex files
         * up front.
         */
        for (File file : files) {
            File zip = null;
            File dir = new File("");
            DexFile dex = null;
            String path = file.getPath();
            String name = file.getName();

            if (path.contains(zipSeparator)) {
                String split[] = path.split(zipSeparator, 2);
                zip = new File(split[0]);
                dir = new File(split[1]);
            } else if (file.isDirectory()) {
                // We support directories for looking up resources and native libraries.
                // Looking up resources in directories is useful for running libcore tests.
                elements.add(new Element(file, true, null, null));
            } else if (file.isFile()) {
                if (name.endsWith(DEX_SUFFIX)) {//原始的dex文件處理，而不是在zip或者jar中的dex文件                  
                    try {
                        dex = loadDexFile(file, optimizedDirectory);
                    } catch (IOException ex) {
                        System.logE("Unable to load dex file: " + file, ex);
                    }
                } else {//處理zip或者jar包中的dex文件
                    zip = file;
                    try {
                        dex = loadDexFile(file, optimizedDirectory);
                    } 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);
                    }
                }
            } else {
                System.logW("ClassLoader referenced unknown path: " + file);
            }

            if ((zip != null) || (dex != null)) {
                elements.add(new Element(dir, false, zip, dex));
            }
        }

        return elements.toArray(new Element[elements.size()]);
    }

dex文件、zip或者jar包中的dex文件都會調用loadDexFile。

private static DexFile loadDexFile(File file, File optimizedDirectory)
            throws IOException {
        if (optimizedDirectory == null) {
            return new DexFile(file);
        } else {
            String optimizedPath = optimizedPathFor(file, optimizedDirectory);
            return DexFile.loadDex(file.getPath(), optimizedPath, 0);
        }
    }

若是optimizedDirectory爲null就會新建一個DexFile對象，調用DexFile的構造函數。

位置：~/android6.0.1_r66/libcore/dalvik/src/main/java/dalvik/system/DexFile.java

...
 public DexFile(File file) throws IOException {
        this(file.getPath());
    }
 
public DexFile(String fileName) throws IOException {//fileName就是上一個構造函數的file.getPath()
        mCookie = openDexFile(fileName, null, 0);
        mFileName = fileName;
        guard.open("close");
}    
...

關鍵函數是openDexFile。

private static Object openDexFile(String sourceName, String outputName, int flags) throws IOException {
        // Use absolute paths to enable the use of relative paths when testing on host.
        return openDexFileNative(new File(sourceName).getAbsolutePath(),
                                 (outputName == null) ? null : new File(outputName).getAbsolutePath(),
                                 flags);
    }

openDexFileNative是一個native函數，須要找到具體實現：
grep -rns：-r指定要查找的是目錄 -n顯示行號 -s不顯示錯誤信息



因此openDexFileNative的具體實現就是：

位置：~/android6.0.1_r66/art/runtime/native/dalvik_system_DexFile.cc

static jobject DexFile_openDexFileNative(
    JNIEnv* env, jclass, jstring javaSourceName, jstring javaOutputName, jint) {
  ScopedUtfChars sourceName(env, javaSourceName);
  if (sourceName.c_str() == nullptr) {
    return 0;
  }
  NullableScopedUtfChars outputName(env, javaOutputName);
  if (env->ExceptionCheck()) {
    return 0;
  }

  ClassLinker* linker = Runtime::Current()->GetClassLinker();
  std::vector<std::unique_ptr<const DexFile>> dex_files;
  std::vector<std::string> error_msgs;

  dex_files = linker->OpenDexFilesFromOat(sourceName.c_str(), outputName.c_str(), &error_msgs);

  if (!dex_files.empty()) {
    jlongArray array = ConvertNativeToJavaArray(env, dex_files);
    if (array == nullptr) {
      ScopedObjectAccess soa(env);
      for (auto& dex_file : dex_files) {
        if (Runtime::Current()->GetClassLinker()->IsDexFileRegistered(*dex_file)) {
          dex_file.release();
        }
      }
    }
    return array;
  } else {
    ScopedObjectAccess soa(env);
    CHECK(!error_msgs.empty());
    // The most important message is at the end. So set up nesting by going forward, which will
    // wrap the existing exception as a cause for the following one.
    auto it = error_msgs.begin();
    auto itEnd = error_msgs.end();
    for ( ; it != itEnd; ++it) {
      ThrowWrappedIOException("%s", it->c_str());
    }

    return nullptr;
  }
}