classloader 是采用双亲委派的方式加载所需要的类。
protected Class loadClass(String var1, boolean var2) throws ClassNotFoundException {
synchronized(this.getClassLoadingLock(var1)) {
Class var4 = this.findLoadedClass(var1);
if(var4 == null) {
...
try {
if(this.parent != null) {
var4 = this.parent.loadClass(var1, false);
} else {
var4 = this.findBootstrapClassOrNull(var1);
}
} catch (ClassNotFoundException var10) {
;
}
if(var4 == null) {
...
var4 = this.findClass(var1);
....
}
}
if(var2) {
this.resolveClass(var4);
}
return var4;
}
}
双亲委派：从classloader的源码分析，在加载类的时候，先在自己已加载的类中查找该类，加载过了就直接返回；如果没有找到就调用父classloader的loadClass方法，如果找到了就直接返回，以此类推，直到
从 bootstrapclass 中加载。如果仍未找到就调用自身的findClass查找相关的类，这是个待实现的方法，需要具体的 classloader 子类自己实现。
Android 中 classloader 大致分以下几种。
BootClassLoader ，PathClassLoader 和 DexClassLoader
// 位于类加载器链的头部，负责将请求委托给VM内部类加载机制
BootClassLoader (PathClassLoader的父classloader)
// 系统类加载器，是新类加载器的 parent，通常用于启动应用程序时。
->PathClassLoader (在ApplicationLoaders.getClassLoader()中，调用ClassLoader.getSystemClassLoader()时返回)
1、PathClassLoader 、DexClassLoader的源码：
// DexClassLoader.java
public class DexClassLoader extends BaseDexClassLoader {
public DexClassLoader(String dexPath, String optimizedDirectory,
String libraryPath, ClassLoader parent) {
super(dexPath, new File(optimizedDirectory), libraryPath, parent);
}
}
// PathClassLoader.java
public class PathClassLoader extends BaseDexClassLoader {
public PathClassLoader(String dexPath, ClassLoader parent) {
super(dexPath, null, null, parent);
}
public PathClassLoader(String dexPath, String libraryPath,
ClassLoader parent) {
super(dexPath, null, libraryPath, parent);
}
}
从以上源码可以看到，两个类很相似，都集成自 BaseDexClassLoader，细微之处就是 PathClassLoader 没有传 optimizedDirectory 这个参数。
2、 BaseDexClassLoader 的源码：
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);
}
@Override
protected Class findClass(String name) throws ClassNotFoundException {
List suppressedExceptions = new ArrayList();
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;
}
...
}
从以上代码发现 findClass 的实现是在 DexPathList 中查找相关的类。
DexPathList 的关键源码：
/*package*/ final class DexPathList {
private static final String DEX_SUFFIX = ".dex";
private static final String JAR_SUFFIX = ".jar";
private static final String ZIP_SUFFIX = ".zip";
private static final String APK_SUFFIX = ".apk";
...
/**
* List of dex/resource (class path) elements.
* Should be called pathElements, but the Facebook app uses reflection
* to modify 'dexElements' (http://b/7726934).
*/
private final Element[] dexElements;
....
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 suppressedExceptions = new ArrayList();
this.dexElements = makeDexElements(splitDexPath(dexPath), optimizedDirectory,
suppressedExceptions);
if (suppressedExceptions.size() > 0) {
this.dexElementsSuppressedExceptions =
suppressedExceptions.toArray(new IOException[suppressedExceptions.size()]);
} else {
dexElementsSuppressedExceptions = null;
}
this.nativeLibraryDirectories = splitLibraryPath(libraryPath);
}
...
/**
* 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. (That is, directories are not included in the
* result.)
*/
private static ArrayList splitDexPath(String path) {
return splitPaths(path, null, false);
}
/**
* Helper for {@link #splitPaths}, which does the actual splitting
* and filtering and adding to a result.
*/
private static void splitAndAdd(String searchPath, boolean directoriesOnly,
ArrayList resultList) {
if (searchPath == null) {
return;
}
for (String path : searchPath.split(":")) {
try {
StructStat sb = Libcore.os.stat(path);
if (!directoriesOnly || S_ISDIR(sb.st_mode)) {
resultList.add(new File(path));
}
} catch (ErrnoException ignored) {
}
}
}
...
/**
* Makes an array of dex/resource path elements, one per element of
* the given array.
*/
private static Element[] makeDexElements(ArrayList files, File optimizedDirectory,
ArrayList suppressedExceptions) {
ArrayList elements = new ArrayList();
/*
* Open all files and load the (direct or contained) dex files
* up front.
*/
for (File file : files) {
File zip = null;
DexFile dex = null;
String name = file.getName();
if (name.endsWith(DEX_SUFFIX)) {
// Raw dex file (not inside a zip/jar).
try {
dex = loadDexFile(file, optimizedDirectory);
} catch (IOException ex) {
System.logE("Unable to load dex file: " + file, ex);
}
} else if (name.endsWith(APK_SUFFIX) || name.endsWith(JAR_SUFFIX)
|| name.endsWith(ZIP_SUFFIX)) {
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 if (file.isDirectory()) {
// We support directories for looking up resources.
// This is only useful for running libcore tests.
elements.add(new Element(file, true, null, null));
} else {
System.logW("Unknown file type for: " + file);
}
if ((zip != null) || (dex != null)) {
elements.add(new Element(file, false, zip, dex));
}
}
return elements.toArray(new Element[elements.size()]);
}
/**
* Constructs a {@code DexFile} instance, as appropriate depending
* on whether {@code optimizedDirectory} is {@code null}.
*/
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);
}
}
很清楚，DexPathList 中 findClass 的策略是：把加载的dex，jar等文件转换为 Element 对象存放到ArrayList中，然后在这些 Elements 中依次查找所需要的类。