一、什么是65535限制？
众所周知，随着Android平台持续增长，Android应用程序越来越大。在Android系统中，一个App的所有代码都在一个Dex文件里面。Dex是一个类似Jar的存储了多有Java编译字节码的归档文件。因为Android系统使用Dalvik虚拟机，所以需要把使用Java Compiler编译之后的class文件转换成Dalvik能够执行的class文件。这里需要强调的是，Dex和Jar一样是一个归档文件，里面仍然是Java代码对应的字节码文件。当Android系统启动一个应用的时候，有一步是对Dex进行优化，这个过程有一个专门的工具来处理，叫DexOpt。DexOpt的执行过程是在第一次加载Dex文件的时候执行的。这个过程会生成一个ODEX文件，即Optimised。DexOpt会把每一个类的方法id检索起来，存在一个链表结构里面。但是这个链表的长度是用一个short类型来保存的，导致了方法id的数目不能够超过65536个。
65536是什么样的数？2的16次方或者16进制的 0xFFFF
下边这个error是不是很熟悉
较高版本的Android构建系统下的提示（Android 7.0及以下）：
Conversion to Dalvik format failed:
Unable to execute dex: method ID not in [0, 0xffff]: 65536
较高版本的Android构建系统的报错信息(Android 8.0)
trouble writing output:
Too many field references: 131000; max is 65536.
You may try using --multi-dex option.二、为什么会出现64K的限制呢？
一般排查问题我们需要从问题本身入手，那么log是最重要的信息。
在构建流程中出现这种问题，根据提示我们大概明白方法数过大，而这些方法是存在于编译后的.class文件中的，而.class最后要存在于dex文件中。
那么如此分析的话，问题应该存在于dex的打包流程当中，这个需要以后深入了解一下。
根据前人的一些分析，我们来看看MemberIdsSection文件。 注意： 源码路径是 /dalvik/dx/src/com/android/dx/dex/file/MemberIdsSection.java 不是/dalvik/dexgen/src/com/android/dexgen/dex/file/MemberIdsSection.java 代码不多，如下：
/*
* Copyright (C) 2007 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.android.dx.dex.file;
import com.android.dex.DexFormat;
import com.android.dex.DexIndexOverflowException;
import java.util.Formatter;
import java.util.Map;
import java.util.TreeMap;
import java.util.concurrent.atomic.AtomicInteger;
/**
* Member (field or method) refs list section of a {@code .dex} file.
*/
public abstract class MemberIdsSection extends UniformItemSection {
/**
* Constructs an instance. The file offset is initially unknown.
*
* @param name {@code null-ok;} the name of this instance, for annotation
* purposes
* @param file {@code non-null;} file that this instance is part of
*/
public MemberIdsSection(String name, DexFile file) {
super(name, file, 4);
}
/** {@inheritDoc} */
@Override
protected void orderItems() {
int idx = 0;
if (items().size() > DexFormat.MAX_MEMBER_IDX + 1) {
throw new DexIndexOverflowException(getTooManyMembersMessage());
}
for (Object i : items()) {
((MemberIdItem) i).setIndex(idx);
idx++;
}
}
private String getTooManyMembersMessage() {
Map<String, AtomicInteger> membersByPackage = new TreeMap<String, AtomicInteger>();
for (Object member : items()) {
String packageName = ((MemberIdItem) member).getDefiningClass().getPackageName();
AtomicInteger count = membersByPackage.get(packageName);
if (count == null) {
count = new AtomicInteger();
membersByPackage.put(packageName, count);
}
count.incrementAndGet();
}
Formatter formatter = new Formatter();
try {
String memberType = this instanceof MethodIdsSection ? "method" : "field";
formatter.format("Too many %1$s references to fit in one dex file: %2$d; max is %3$d.%n" +
"You may try using multi-dex. If multi-dex is enabled then the list of " +
"classes for the main dex list is too large.%n" +
"References by package:",
memberType, items().size(), DexFormat.MAX_MEMBER_IDX + 1);
for (Map.Entry<String, AtomicInteger> entry : membersByPackage.entrySet()) {
formatter.format("%n%6d %s", entry.getValue().get(), entry.getKey());
}
return formatter.toString();
} finally {
formatter.close();
}
}
}
在48行到49中，我们看到如下可能抛出异常的情况
if (items().size() > DexFormat.MAX_MEMBER_IDX + 1) {
throw new DexIndexOverflowException(getTooManyMembersMessage());
}
getTooManyMembersMessage()函数内(72行到77行)有如下异常信息字符串构造
String memberType = this instanceof MethodIdsSection ? "method" : "field";
formatter.format("Too many %1$s references to fit in one dex file: %2$d; max is %3$d.%n" +
"You may try using multi-dex. If multi-dex is enabled then the list of " +
"classes for the main dex list is too large.%n" +
"References by package:",
memberType, items().size(), DexFormat.MAX_MEMBER_IDX + 1);同时我们还要注意DexFormat类
/**
* Maximum addressable field or method index.
* The largest addressable member is 0xffff, in the "instruction formats" spec as field@CCCC or
* meth@CCCC.
*/
public static final int MAX_MEMBER_IDX = 0xFFFF;根据注释,我们来到Dalvik 字节码，根据表格中的解释如下图：
可以看到类型索引（16 位），由此可以知道，无论是方法数还是字段数都不能超过65536，这也就是为什么在构建流程中出现65536的报错信息。
由此可以得出结论：
invoke-kind （调用各类方法）指令中，方法引用索引数是 16 位的，也就是最多调用 2^16 = 65536 个方法，这就是 DexFormat 中 MAX_MEMBER_IDX 为 0xFFFF 的原因。 所以单个dex的方法或者字段数量不能超过65536
三、Android 官方是如何解决65536问题的？
官方提出了通过multidex包进行多dex编译的方法
1.导入multidex包，设置为支持多dex输出模式
android {
compileSdkVersion 26
buildToolsVersion "26.1.0"
defaultConfig {
...
minSdkVersion 14
targetSdkVersion 26
...
// Enabling multidex support.
multiDexEnabled true
}
...
}
afterEvaluate {
tasks.matching {
it.name.startsWith('dex')
}.each { dx ->
if (dx.additionalParameters == null) {
dx.additionalParameters = []
}
dx.additionalParameters += '--multi-dex' // enable multidex
// optional
// dx.additionalParameters += "--main-dex-list=$projectDir/<filename>".toString() // enable the main-dex-list
}
}
dependencies {
compile 'com.android.support:multidex:1.0.1'
}
2.覆写Application类
如果你的工程中已经含有Application类,那么让它继承android.support.multidex.MultiDexApplication类,
如果你的Application已经继承了其他类并且不想做改动，那么还有另外一种使用方式,覆写attachBaseContext()方法。
import android.support.multidex.MultiDex;
@Override
protected void attachBaseContext(Context base) {
super.attachBaseContext(base);
MultiDex.install(this);
}
这样处理后，若代码函数超过65535, 就会生成多个dex文件。不会再报错。
四、MultiDex 对App的影响和解决方案
MultiDex分包引起的app首次启动过慢：默认情况下，Dalvik会限制app只有一个class.dex字节码文件,multidex则可以取消这个限制。multidex会成为主dex文件的一部分，然后对其他分dex文件及其所访问的代码进行管理。在app首次启动的时候，主dex文件会进行复杂的计算来确定主dex文件所应该包含那些class文件，引起app首次启动过慢的原因就这里。
解决这个问题需要引入multidex.keep文件，来告诉主dex文件首次启动时不需要计算主dex包含的class文件，直接取multidex.keep列出来的文件即可。需要如下步骤，
1、在build.gradle同目录中新建multidex.keep文件
2、在build.gradle文件中指明主dex文件该包含的class文件应该从multidex.keep文件获取
android.applicationVariants.all { variant ->
task "fix${variant.name.capitalize()}MainDexClassList" << {
logger.info "Fixing main dex keep file for $variant.name"
File keepFile = new File("$buildDir/intermediates/multi-dex/$variant.buildType.name/maindexlist.txt")
keepFile.withWriterAppend { w ->
// Get a reader for the input file
w.append('\n')
new File("${projectDir}/multidex.keep").withReader { r ->
// And write data from the input into the output
w << r << '\n'
}
logger.info "Updated main dex keep file for ${keepFile.getAbsolutePath()}\n$keepFile.text"
}
}
}
tasks.whenTaskAdded { task ->
android.applicationVariants.all { variant ->
if (task.name == "create${variant.name.capitalize()}MainDexClassList") {
task.finalizedBy "fix${variant.name.capitalize()}MainDexClassList"
}
}
}3.在multidex.keep文件中添加主dex文件所需的class文件。主dex文件所需的class文件可以通过以下方法获取。调用MultiDexUtils的getLoadedExternalDexClasses方法即可获取所需的class文件的list。
import android.content.Context;
import android.content.SharedPreferences;
import android.content.pm.ApplicationInfo;
import android.content.pm.PackageManager;
import android.os.Build;
import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Enumeration;
import java.util.List;
import dalvik.system.DexFile;
public class MultiDexUtils {
private static final String EXTRACTED_NAME_EXT = ".classes";
private static final String EXTRACTED_SUFFIX = ".zip";
private static final String SECONDARY_FOLDER_NAME = "code_cache" + File.separator +
"secondary-dexes";
private static final String PREFS_FILE = "multidex.version";
private static final String KEY_DEX_NUMBER = "dex.number";
private SharedPreferences getMultiDexPreferences(Context context) {
return context.getSharedPreferences(PREFS_FILE,
Build.VERSION.SDK_INT < Build.VERSION_CODES.HONEYCOMB
? Context.MODE_PRIVATE
: Context.MODE_PRIVATE | Context.MODE_MULTI_PROCESS);
}
/**
* get all the dex path
*
* @param context the application context
* @return all the dex path
* @throws PackageManager.NameNotFoundException
* @throws IOException
*/
public List<String> getSourcePaths(Context context) throws PackageManager.NameNotFoundException, IOException {
final ApplicationInfo applicationInfo = context.getPackageManager().getApplicationInfo(context.getPackageName(), 0);
final File sourceApk = new File(applicationInfo.sourceDir);
final File dexDir = new File(applicationInfo.dataDir, SECONDARY_FOLDER_NAME);
final List<String> sourcePaths = new ArrayList<>();
sourcePaths.add(applicationInfo.sourceDir); //add the default apk path
//the prefix of extracted file, ie: test.classes
final String extractedFilePrefix = sourceApk.getName() + EXTRACTED_NAME_EXT;
//the total dex numbers
final int totalDexNumber = getMultiDexPreferences(context).getInt(KEY_DEX_NUMBER, 1);
for (int secondaryNumber = 2; secondaryNumber <= totalDexNumber; secondaryNumber++) {
//for each dex file, ie: test.classes2.zip, test.classes3.zip...
final String fileName = extractedFilePrefix + secondaryNumber + EXTRACTED_SUFFIX;
final File extractedFile = new File(dexDir, fileName);
if (extractedFile.isFile()) {
sourcePaths.add(extractedFile.getAbsolutePath());
//we ignore the verify zip part
} else {
throw new IOException("Missing extracted secondary dex file '" +
extractedFile.getPath() + "'");
}
}
return sourcePaths;
}
/**
* get all the external classes name in "classes2.dex", "classes3.dex" ....
*
* @param context the application context
* @return all the classes name in the external dex
* @throws PackageManager.NameNotFoundException
* @throws IOException
*/
public List<String> getExternalDexClasses(Context context) throws PackageManager.NameNotFoundException, IOException {
final List<String> paths = getSourcePaths(context);
if(paths.size() <= 1) {
// no external dex
return null;
}
// the first element is the main dex, remove it.
paths.remove(0);
final List<String> classNames = new ArrayList<>();
for (String path : paths) {
try {
DexFile dexfile = null;
if (path.endsWith(EXTRACTED_SUFFIX)) {
//NOT use new DexFile(path), because it will throw "permission error in /data/dalvik-cache"
dexfile = DexFile.loadDex(path, path + ".tmp", 0);
} else {
dexfile = new DexFile(path);
}
final Enumeration<String> dexEntries = dexfile.entries();
while (dexEntries.hasMoreElements()) {
classNames.add(dexEntries.nextElement());
}
} catch (IOException e) {
throw new IOException("Error at loading dex file '" +
path + "'");
}
}
return classNames;
}
/**
* Get all loaded external classes name in "classes2.dex", "classes3.dex" ....
* @param context
* @return get all loaded external classes
*/
public List<String> getLoadedExternalDexClasses(Context context) {
try {
final List<String> externalDexClasses = getExternalDexClasses(context);
if (externalDexClasses != null && !externalDexClasses.isEmpty()) {
final ArrayList<String> classList = new ArrayList<>();
final java.lang.reflect.Method m = ClassLoader.class.getDeclaredMethod("findLoadedClass", new Class[]{String.class});
m.setAccessible(true);
final ClassLoader cl = context.getClassLoader();
for (String clazz : externalDexClasses) {
if (m.invoke(cl, clazz) != null) {
classList.add(clazz.replaceAll("\\.", "/").replaceAll("#34;, ".class"));
}
}
return classList;
}
} catch (Exception e) {
e.printStackTrace();
}
return null;
}
}4.再把list数组写到手机sd卡的txt文件中，把txt文件的内容复制到multidex.keep即可。
MultiDexUtils dexUtils = new MultiDexUtils();
List<String> des = dexUtils.getLoadedExternalDexClasses(this);
String sdCardDir = Environment.getExternalStorageDirectory().getAbsolutePath();
File saveFile = new File(sdCardDir, "aaaa.txt");
try {
FileOutputStream outStream = new FileOutputStream(saveFile);
outStream.write(listToString(des).getBytes());
outStream.close();
} catch (IOException e) {
e.printStackTrace();
}public static String listToString(List<String> stringList){
if(stringList==null) {
return null;
}
StringBuilder result = new StringBuilder();
boolean flag=false;
for(String string : stringList) {
if(flag) {
result.append("\n");
}else{
flag=true;
}
result.append(string);
}
return result.toString();
}以上是关于65535问题和解决方案的总结，欢迎大家转发留言进行交流。
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