Android为什么会有65536的方法数量限制

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前言

65536是什么样的数？2的16次方或者说64KB

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.

注意：构建时期发生的错误哦

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。代码不多，如下：

1 /*
2 * Copyright (C) 2007 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 *      http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17package com.android.dx.dex.file;
18
19import com.android.dex.DexFormat;
20import com.android.dex.DexIndexOverflowException;
21
22import java.util.Formatter;
23import java.util.Map;
24import java.util.TreeMap;
25import java.util.concurrent.atomic.AtomicInteger;
26
27/**
28 * Member (field or method) refs list section of a {@code .dex} file.
29 */
30public abstract class MemberIdsSection extends UniformItemSection {
31
32    /**
33     * Constructs an instance. The file offset is initially unknown.
34     *
35     * @param name {@code null-ok;} the name of this instance, for annotation
36     * purposes
37     * @param file {@code non-null;} file that this instance is part of
38     */
39    public MemberIdsSection(String name, DexFile file) {
40        super(name, file, 4);
41    }
42
43    /** {@inheritDoc} */
44    @Override
45    protected void orderItems() {
46        int idx = 0;
47
48        if (items().size() > DexFormat.MAX_MEMBER_IDX + 1) {
49            throw new DexIndexOverflowException(getTooManyMembersMessage());
50        }
51
52        for (Object i : items()) {
53            ((MemberIdItem) i).setIndex(idx);
54            idx++;
55        }
56    }
57
58    private String getTooManyMembersMessage() {
59        Map membersByPackage = new TreeMap();
60        for (Object member : items()) {
61            String packageName = ((MemberIdItem) member).getDefiningClass().getPackageName();
62            AtomicInteger count = membersByPackage.get(packageName);
63            if (count == null) {
64                count = new AtomicInteger();
65                membersByPackage.put(packageName, count);
66            }
67            count.incrementAndGet();
68        }
69
70        Formatter formatter = new Formatter();
71        try {
72            String memberType = this instanceof MethodIdsSection ? "method" : "field";
73            formatter.format("Too many %1$s references to fit in one dex file: %2$d; max is %3$d.%n" +
74                            "You may try using multi-dex. If multi-dex is enabled then the list of " +
75                            "classes for the main dex list is too large.%n" +
76                    "References by package:",
77                    memberType, items().size(), DexFormat.MAX_MEMBER_IDX + 1);
78            for (Map.Entry entry : membersByPackage.entrySet()) {
79                formatter.format("%n%6d %s", entry.getValue().get(), entry.getKey());
80            }
81            return formatter.toString();
82        } finally {
83            formatter.close();
84        }
85    }
86
87}

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：

单个dex的方法或者字段数量不能超过65536

如何避免64K问题？涉及到dex分包的知识，同时也是涉及到APK瘦身优化等问题。

很多问题都需要我们去直面，一步步去解决。