由Android 65K方法数限制引发的思考 - 开发者俱乐部

65536网上众说纷纭，有对的，有不全对的，也有错的。下面将跟踪最新的AOSP源码来顺藤摸瓜，但是探索问题必然迂回冗余，仅作记录，读者可直接跳过看结果。

/*
* Direct-mapped "header_item" struct.
*/
struct DexHeader {
u1 magic[8];
u4 checksum;
u1 signature[kSHA1DigestLen];
u4 fileSize;
u4 headerSize;
u4 endianTag;
u4 linkSize;
u4 linkOff;
u4 mapOff;
u4 stringIdsSize;
u4 stringIdsOff;
u4 typeIdsSize;
u4 typeIdsOff;
u4 protoIdsSize;
u4 protoIdsOff;
u4 fieldIdsSize;
u4 fieldIdsOff;
u4 methodIdsSize; // 这里存放了方法字段索引的大小，methodIdsSize的类型为u4
u4 methodIdsOff;
u4 classDefsSize;
u4 classDefsOff;
u4 dataSize;
u4 dataOff;
};

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/*
* These match the definitions in the VM specification.
*/
typedef uint8_t u1;
typedef uint16_t u2;
typedef uint32_t u4;
typedef uint64_t u8;
typedef int8_t s1;
typedef int16_t s2;
typedef int32_t s4;
typedef int64_t s8;

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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 Dex。执行ODex的效率会比直接执行Dex文件的效率要高很多。但是在早期的Android系统中，DexOpt有一个问题，也就是这篇文章想要说明并解决的问题。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。当一个项目足够大的时候，显然这个方法数的上限是不够的。尽管在新版本的Android系统中，DexOpt修复了这个问题，但是我们仍然需要对老系统做兼容。

只能在dalvik目录下搜索关键字”methid ID not in”，在DexMergger里找到了抛出异常的地方：

/**
* Combine two dex files into one.
*/
public final class DexMerger {
private void mergeMethodIds() {
new IdMerger<MethodId>(idsDefsOut) {
@Override TableOfContents.Section getSection(TableOfContents tableOfContents) {
return tableOfContents.methodIds;
}
@Override MethodId read(Dex.Section in, IndexMap indexMap, int index) {
return indexMap.adjust(in.readMethodId());
}
@Override void updateIndex(int offset, IndexMap indexMap, int oldIndex, int newIndex) {
if (newIndex < 0 || newIndex > 0xffff) {
throw new DexIndexOverflowException(
"method ID not in [0, 0xffff]: " + newIndex);
}
indexMap.methodIds[oldIndex] = (short) newIndex;
}
@Override void write(MethodId methodId) {
methodId.writeTo(idsDefsOut);
}
}.mergeSorted();
}
}

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这里定义了indexMap的methodIds的单项值要强转short，所以在存放之前check一下范围是不是0 ~ 0xffff。我们看看IndexMap的定义：

/**
* Maps the index offsets from one dex file to those in another. For example, if
* you have string #5 in the old dex file, its position in the new dex file is
* {@code strings[5]}.
*/
public final class IndexMap {
private final Dex target;
public final int[] stringIds;
public final short[] typeIds;
public final short[] protoIds;
public final short[] fieldIds;
public final short[] methodIds;
// ... ...
}

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看上去是对了，可是这个DexMerger是合并两个dex的，默认情况下我们只有一个dex的，那么这个65k是哪里限制的呢？再查！

// 源码目录：dalvik/dx
// Main.java
-> main() -> run() -> runMonoDex()(或者runMultiDex()) -> writeDex()
// DexFile
-> toDex() -> toDex0()
// MethodIdsSection extends MemberIdsSection extends UniformItemSection extends Section
-> prepare() -> prepare0() -> orderItems() -> getTooManyMembersMessage()
// Main.java
-> getTooManyIdsErrorMessage()

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package com.android.dx.dex.file;
import com.android.dex.DexException;
import com.android.dex.DexFormat;
import com.android.dex.DexIndexOverflowException;
import com.android.dx.command.dexer.Main;
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 %s references: %d; max is %d.%n" +
Main.getTooManyIdsErrorMessage() + "%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();
}
}
}

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// 如果方法数大于0xffff就提示65k错误
if (items().size() > DexFormat.MAX_MEMBER_IDX + 1) {
throw new DexIndexOverflowException(getTooManyMembersMessage());
}
// 这个DexFormat.MAX_MEMBER_IDX就是0xFFFF
/**
* 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;

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