classloader 是采用雙親委派的方式加載所需要的類。
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;
}
}
雙親委派：從classloader的源碼分析摆霉，在加載類的時候漓帚，先在自己已加載的類中查找該類蹬敲，加載過了就直接返回；如果沒有找到就調(diào)用父classloader的loadClass方法舶得，如果找到了就直接返回掰烟，以此類推，直到
從 bootstrapclass 中加載沐批。如果仍未找到就調(diào)用自身的findClass查找相關(guān)的類纫骑，這是個待實現(xiàn)的方法，需要具體的 classloader 子類自己實現(xiàn)九孩。
Android 中 classloader 大致分以下幾種先馆。
BootClassLoader ，PathClassLoader 和 DexClassLoader
// 位于類加載器鏈的頭部捻撑，負(fù)責(zé)將請求委托給VM內(nèi)部類加載機制
BootClassLoader (PathClassLoader的父classloader)
// 系統(tǒng)類加載器磨隘，是新類加載器的 parent，通常用于啟動應(yīng)用程序時顾患。
->PathClassLoader (在ApplicationLoaders.getClassLoader()中番捂，調(diào)用ClassLoader.getSystemClassLoader()時返回)
1、PathClassLoader 江解、DexClassLoader的源碼：
// 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);
}
}
從以上源碼可以看到设预，兩個類很相似，都集成自 BaseDexClassLoader犁河，細(xì)微之處就是 PathClassLoader 沒有傳 optimizedDirectory 這個參數(shù)鳖枕。
2、 BaseDexClassLoader 的源碼：
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<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;
}
...
}
從以上代碼發(fā)現(xiàn) findClass 的實現(xiàn)是在 DexPathList 中查找相關(guān)的類桨螺。
DexPathList 的關(guān)鍵源碼：
/*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<IOException> suppressedExceptions = new ArrayList<IOException>();
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<File> 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<File> 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<File> files, File optimizedDirectory,
ArrayList<IOException> suppressedExceptions) {
ArrayList<Element> elements = new ArrayList<Element>();
/*
* 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);
}
}
很清楚宾符，DexPathList 中 findClass 的策略是：把加載的dex，jar等文件轉(zhuǎn)換為 Element 對象存放到ArrayList中灭翔，然后在這些 Elements 中依次查找所需要的類魏烫。
