任何一個java程序都是有一個或多個class文件組成的，在程序運行時，需要將class文件加載到JVM虛擬機中才可以用，負責加載這些class文件的就是java的類加載機制。
簡單來說：就是加載class文件，提供給程序運行時使用。
知識點
1）基本職責就是根據一個指定的類的名稱，找到或者生成其對應的字節(jié)碼，然后從這些字節(jié)碼代碼中定義出一個java類的實例。
2）工作原理則是通過雙親委托機制來實現的，這也可以避免重復加載，保證類的唯一性，且java類隨著他的類加載器一起具備了一種帶有優(yōu)先級的層次關系。
? 如何保證類的唯一性呢？
JVM在判斷兩個class是否相同時，不僅僅要判斷兩個類名是否相同，還要判斷負責加載這兩個類的加載器是否一樣，也就是說同一個類加載器加載相同名稱的類，虛擬機才會認為加載的是同一個類，否則的話認為他們是不同的類。
ClassLoader是一個抽象類，他在android中的具體實現類有：
**1. BootClassLoader:** 用于加載android Framework層的class文件
**2. PathClassLoader:** 用于加載android 應用程序中的類加載器，可以是指定的dex，jar, zip , apk中的class.dex
**3. DexClassLoader:** 用于加載指定的dex ,jar ,zip ,apk中的classes.dex
類加載器類圖
了解上面的一些基礎概念理論的東西后，我們用一張流程圖來分析一下類加載器的工作原理：
類加載的工作原理流程圖
知識點：
類加載的雙親委托機制
? 類加載器在嘗試自己去查找某一個類的字節(jié)碼時，會先就這個任務交給他的父類加載器去嘗試加載這個類，而父類加載器也先交給父類的父類加載器去執(zhí)行，依次遞歸查找。如果父類找到了則直接返回，如果父類沒有找到則再由類加載器自己查找，這種查找的方式就是雙親委托機制。
剖析源碼
? 解析來我們來從源碼角度一步一步潛入類加載器中學習一下，其他類加載器的幾個類都比較簡單，相對復雜的是在ClassLoader.java中。廢話不多說，直接開擼。。。。。。。
-
BaseDexClassLoader.java
public class BaseDexClassLoader extends ClassLoader {
32
33 /**
34 * Hook for customizing how dex files loads are reported.
35 *
36 * This enables the framework to monitor the use of dex files. The
37 * goal is to simplify the mechanism for optimizing foreign dex files and
38 * enable further optimizations of secondary dex files.
39 *
40 * The reporting happens only when new instances of BaseDexClassLoader
41 * are constructed and will be active only after this field is set with
42 * {@link BaseDexClassLoader#setReporter}.
43 */
44 /* @NonNull */ private static volatile Reporter reporter = null;
45
46 private final DexPathList pathList;
47
48 /**
49 * Constructs an instance.
50 * Note that all the *.jar and *.apk files from {@code dexPath} might be
51 * first extracted in-memory before the code is loaded. This can be avoided
52 * by passing raw dex files (*.dex) in the {@code dexPath}.
53 *
54 * @param dexPath the list of jar/apk files containing classes and
55 * resources, delimited by {@code File.pathSeparator}, which
56 * defaults to {@code ":"} on Android.
57 * @param optimizedDirectory this parameter is deprecated and has no effect since API level 26.
58 * @param librarySearchPath the list of directories containing native
59 * libraries, delimited by {@code File.pathSeparator}; may be
60 * {@code null}
61 * @param parent the parent class loader
62 */
63 public BaseDexClassLoader(String dexPath, File optimizedDirectory,
64 String librarySearchPath, ClassLoader parent) {
65 this(dexPath, optimizedDirectory, librarySearchPath, parent, false);
66 }
67
68 /**
69 * @hide
70 */
71 public BaseDexClassLoader(String dexPath, File optimizedDirectory,
72 String librarySearchPath, ClassLoader parent, boolean isTrusted) {
73 super(parent);
//存放dex的路徑
74 this.pathList = new DexPathList(this, dexPath, librarySearchPath, null, isTrusted);
75
76 if (reporter != null) {
77 reportClassLoaderChain();
78 }
79 }
80
81
113 /**
114 * Constructs an instance.
115 *
116 * dexFile must be an in-memory representation of a full dexFile.
117 *
118 * @param dexFiles the array of in-memory dex files containing classes.
119 * @param parent the parent class loader
120 *
121 * @hide
122 */
123 public BaseDexClassLoader(ByteBuffer[] dexFiles, ClassLoader parent) {
124 // TODO We should support giving this a library search path maybe.
125 super(parent);
126 this.pathList = new DexPathList(this, dexFiles);
127 }
128
129 @Override
130 protected Class<?> findClass(String name) throws ClassNotFoundException {
131 List<Throwable> suppressedExceptions = new ArrayList<Throwable>();
132 Class c = pathList.findClass(name, suppressedExceptions);
133 if (c == null) {
134 ClassNotFoundException cnfe = new ClassNotFoundException(
135 "Didn't find class \"" + name + "\" on path: " + pathList);
136 for (Throwable t : suppressedExceptions) {
137 cnfe.addSuppressed(t);
138 }
139 throw cnfe;
140 }
141 return c;
142 }
281}
2. PathClassLoader.java
public class PathClassLoader extends BaseDexClassLoader {
26 /**
27 * Creates a {@code PathClassLoader} that operates on a given list of files
28 * and directories. This method is equivalent to calling
29 * {@link #PathClassLoader(String, String, ClassLoader)} with a
30 * {@code null} value for the second argument (see description there).
31 *
32 * @param dexPath the list of jar/apk files containing classes and
33 * resources, delimited by {@code File.pathSeparator}, which
34 * defaults to {@code ":"} on Android
35 * @param parent the parent class loader
36 */
37 public PathClassLoader(String dexPath, ClassLoader parent) {
38 super(dexPath, null, null, parent);
39 }
40
41 /**
42 * Creates a {@code PathClassLoader} that operates on two given
43 * lists of files and directories. The entries of the first list
44 * should be one of the following:
45 *
46 * <ul>
47 * <li>JAR/ZIP/APK files, possibly containing a "classes.dex" file as
48 * well as arbitrary resources.
49 * <li>Raw ".dex" files (not inside a zip file).
50 * </ul>
51 *
52 * The entries of the second list should be directories containing
53 * native library files.
54 *
55 * @param dexPath the list of jar/apk files containing classes and
56 * resources, delimited by {@code File.pathSeparator}, which
57 * defaults to {@code ":"} on Android
58 * @param librarySearchPath the list of directories containing native
59 * libraries, delimited by {@code File.pathSeparator}; may be
60 * {@code null}
61 * @param parent the parent class loader
62 */
63 public PathClassLoader(String dexPath, String librarySearchPath, ClassLoader parent) {
64 super(dexPath, null, librarySearchPath, parent);
65 }
66}
-
DexClassLoader.java
public class DexClassLoader extends BaseDexClassLoader {
36 /**
37 * Creates a {@code DexClassLoader} that finds interpreted and native
38 * code. Interpreted classes are found in a set of DEX files contained
39 * in Jar or APK files.
40 *
41 * <p>The path lists are separated using the character specified by the
42 * {@code path.separator} system property, which defaults to {@code :}.
43 *
44 * @param dexPath the list of jar/apk files containing classes and
45 * resources, delimited by {@code File.pathSeparator}, which
46 * defaults to {@code ":"} on Android
47 * @param optimizedDirectory this parameter is deprecated and has no effect since API level 26.
48 * @param librarySearchPath the list of directories containing native
49 * libraries, delimited by {@code File.pathSeparator}; may be
50 * {@code null}
51 * @param parent the parent class loader
52 */
53 public DexClassLoader(String dexPath, String optimizedDirectory,
54 String librarySearchPath, ClassLoader parent) {
55 super(dexPath, null, librarySearchPath, parent);
56 }
57}
-
ClassLoader.java
protected Class<?> loadClass(String name, boolean resolve)
throws ClassNotFoundException
{
// First, check if the class has already been loaded
//從JVM緩存中已經加載的Class中進行查找，已經加載過，則直接返回
//findLoadedClass返回會先判斷當前的類加載器是否是BootClassLoader？如果不是，那么就將當前的加載器傳遞給VMClassLoader ： 如果是，則給VMClassLoader傳遞空值進去
// 最后再通過native方法在緩存中查找
Class<?> c = findLoadedClass(name);
if (c == null) {
//父類加載器加載
try {
if (parent != null) {
//雙親委托，遞歸交給父類加載器進行加載，
c = parent.loadClass(name, false);
} else {
//此方法返回null
c = findBootstrapClassOrNull(name);
}
} catch (ClassNotFoundException e) {
// ClassNotFoundException thrown if class not found
// from the non-null parent class loader
}
//自己進行加載
if (c == null) {
// If still not found, then invoke findClass in order
// to find the class.
//findClass 系統(tǒng)提供的是會直接拋出異常 throw new ClassNotFoundException(name)
//我們可以在自己的類加載器中重寫findClass方法，來做到我們自己去查找加載類的功能
c = findClass(name);
}
}
return c;
}
以上就是筆者想要和大家分享的關于類加載器的一些內容，那么我們實際工作中該如何使用呢？持續(xù)完善中。。。。。。
