首先要了解到“WritePosting（）是DocumentWriter里面的一个方法”，定位要正确。实现作用：

提供了写入.tvd、.tvf、.tvx文件以及.tis、.tii字典文件的入口，同时写入了.frq、.frx文件。

刚开时候看这段代码，可谓“乱七八糟”，看不明白。等看完利用TermVector方式TermVectorsWriter写入.tvd、.tvf、.tvx文件以及TermInfosWriter写入的字典文件后，茅塞顿开。

方法实现：

private final void writePostings(Posting[] postings, String segment)
throws CorruptIndexException, IOException {
IndexOutput freq = null, prox = null;
TermInfosWriter tis = null; // TermInfosWriter类是与词条的写操作有关的
TermVectorsWriter termVectorWriter = null;
try {
// 打开文件流，为倒排的索引进行存储
freq = directory.createOutput(segment + ".frq"); // 打开segments.frq文件
prox = directory.createOutput(segment + ".prx"); // 打开segments.prx文件
tis = new TermInfosWriter(directory, segment, fieldInfos,termIndexInterval); // 创建一个TermInfosWriter对象
TermInfo ti = new TermInfo(); // 创建一个TermInfo对象，该对象用于在内存中管理词条的
String currentField = null;
boolean currentFieldHasPayloads = false;

for (int i = 0; i < postings.length; i++) { // 遍历Posting数组中的每以个对象
Posting posting = postings[i];

// 检查：是否需要转换成一个新的Field
String termField = posting.term.field();
if (currentField != termField) { // 从Posting数组中获取Field的名称(Strnig类型)如果不为null
// 改变currentField。看是否有需要存储的信息
currentField = termField;
FieldInfo fi = fieldInfos.fieldInfo(currentField); // 根据currentField名称从FieldInfos中找到这个FieldInfo对象
currentFieldHasPayloads = fi.storePayloads;
if (fi.storeTermVector) {
if (termVectorWriter == null) {
termVectorWriter =
new TermVectorsWriter(directory, segment, fieldInfos); // 构造一个TermVectorsWriter对象，该对象与词条向量的写操作相关
termVectorWriter.openDocument();
}
termVectorWriter.openField(currentField); // 根据指定的Field的名称currentField打开一个文件输出流

} else if (termVectorWriter != null) {
termVectorWriter.closeField();
}
}

// 为带有指针的sengments.frq文件和segments.prx文件设置一个入口
ti.set(1, freq.getFilePointer(), prox.getFilePointer(), -1); // ti是一个TernInfo类实例，用来管理词条的
tis.add(posting.term, ti); // tis是一个TermInfosWriter类实例，它的方法为add(Term term, TermInfo ti)，将一个<Term, TermInfo>对加入到其中

// 为segments.frq文件添加一个入口
int postingFreq = posting.freq;
if (postingFreq == 1) // optimize freq=1
freq.writeVInt(1); // set low bit of doc num.
else {
freq.writeVInt(0); // the document number
freq.writeVInt(postingFreq); // frequency in doc
}

int lastPosition = 0; // write positions
int[] positions = posting.positions;
Payload[] payloads = posting.payloads;
int lastPayloadLength = -1;

// 下面是对词条Term的Payload和positions信息进行优化处理，写入输出到索引目录中
// The following encoding is being used for positions and payloads:
// Case 1: current field does not store payloads
// Positions -> <PositionDelta>^freq
// PositionDelta -> VInt
// The PositionDelta is the difference between the current
// and the previous position
// Case 2: current field stores payloads
// Positions -> <PositionDelta, Payload>^freq
// Payload -> <PayloadLength?, PayloadData>
// PositionDelta -> VInt
// PayloadLength -> VInt
// PayloadData -> byte^PayloadLength
// In this case PositionDelta/2 is the difference between
// the current and the previous position. If PositionDelta
// is odd, then a PayloadLength encoded as VInt follows,
// if PositionDelta is even, then it is assumed that the
// length of the current Payload equals the length of the
// previous Payload.
for (int j = 0; j < postingFreq; j++) { // 用希腊字母编码
int position = positions[j];
int delta = position - lastPosition;
if (currentFieldHasPayloads) {
int payloadLength = 0;
Payload payload = null;
if (payloads != null) {
payload = payloads[j];
if (payload != null) {
payloadLength = payload.length;
}
}
if (payloadLength == lastPayloadLength) {
// the length of the current payload equals the length
// of the previous _disibledevent= payloadLength;
}
if (payloadLength > 0) {