1. commons-fileupload框架源碼解析(一)--實(shí)例
2. commons-fileupload框架源碼解析(二)--HTTP
3. commons-fileupload框架源碼解析（三）--ParseRequest
4. commons-fileupload框架源碼解析（四）--FileItemIterator
5. commons-fileupload框架源碼解析（五）--MultipartStream
6. commons-fileupload框架源碼解析（六）--ParameterParser
7. commons-fileupload框架源碼解析（七）--FileCleaningTracker
8. commons-fileupload框架源碼解析（八）--DeferredFileOutputStream

簡(jiǎn)介

DeferredFileOutputStream是一個(gè)根據(jù)設(shè)定的最大內(nèi)存閥值去控制寫入到內(nèi)存中，還是寫入到硬盤中畴蹭。

ThresholdingOutputStream

DeferredFileOutputStream繼承ThresholdOutputStream，ThresholdOutputStream繼承OuptStream,重寫了write相關(guān)方法轧叽。

ThresholdOutputStream.write相關(guān)方法

 @Override
    public void write(final int b) throws IOException
    {
        checkThreshold(1);
        getStream().write(b);
        written++;
    }

    @Override
    public void write(final byte b[]) throws IOException
    {
        checkThreshold(b.length);
        getStream().write(b);
        written += b.length;
    }

  @Override
    public void write(final byte b[], final int off, final int len) throws IOException
    {
        checkThreshold(len);
        getStream().write(b, off, len);
        written += len;
    }

主要是加上了chekThreshold(len)方法罗洗，并調(diào)用getStream()交給另一個(gè)流
處理肛跌，并統(tǒng)計(jì)寫入了多少字節(jié)（written）艺配。

ThresholdOutputStream.checkThreshold(int)

protected void checkThreshold(final int count) throws IOException
    {
        if (!thresholdExceeded && written + count > threshold)
        {
            thresholdExceeded = true;
            thresholdReached();
        }
    }

檢查當(dāng)前的寫入到內(nèi)容流的字節(jié)數(shù)written是否大于閥值threshold，大于的時(shí)候會(huì)調(diào)用thresholdReached()

getStream & thresholdReached

/**
     * Returns the underlying output stream, to which the corresponding
     * <code>OutputStream</code> methods in this class will ultimately delegate.
     * 獲取輸出流
     * @return The underlying output stream.
     *
     * @throws IOException if an error occurs.
     */
    protected abstract OutputStream getStream() throws IOException;


    /**
     * Indicates that the configured threshold has been reached, and that a
     * subclass should take whatever action necessary on this event. This may
     * include changing the underlying output stream.
     * 當(dāng)前的寫入字節(jié)數(shù)written是否大于閥值threshold時(shí)觸發(fā)
     * @throws IOException if an error occurs.
     */
    protected abstract void thresholdReached() throws IOException;

這兩個(gè)是抽象方法衍慎，而DeferredFileOutputStream重寫了,先看getStream

DeferredFileOutputStream.getStream

  @Override
    protected OutputStream getStream() throws IOException
    {
        return currentOutputStream;
    }

返回currentOuputStream转唉，一開始默認(rèn)是內(nèi)存流ByteArrayOutputStream。

DeferredFileOutputStream.thresholdReached

@Override
    protected void thresholdReached() throws IOException
    {
        //如果prefix,suffix,directory不為null稳捆，outputFile不管是否已經(jīng)引用了對(duì)象赠法，都會(huì)重新調(diào)用File.createTempFile(String,String,File)來(lái)
        //創(chuàng)建指定的臨時(shí)文件，并讓后outputFile指向這個(gè)文件
        if (prefix != null) {
            //創(chuàng)建臨時(shí)文件
            outputFile = File.createTempFile(prefix, suffix, directory);
        }
        FileUtils.forceMkdirParent(outputFile);//檢查父級(jí)目錄是否存在，不存在則創(chuàng)建砖织，創(chuàng)建父級(jí)文件目錄
        final FileOutputStream fos = new FileOutputStream(outputFile);
        try {
            memoryOutputStream.writeTo(fos);//將已經(jīng)內(nèi)容流的數(shù)據(jù)寫入進(jìn)文件流
        } catch (final IOException e){
            fos.close();//當(dāng)前出現(xiàn)異常時(shí)關(guān)閉流款侵，并拋出異常
            throw e;
        }
        currentOutputStream = fos;
        memoryOutputStream = null;
    }

當(dāng)達(dá)到閥值的時(shí)候，thresholdReached()就會(huì)被調(diào)用侧纯，會(huì)將內(nèi)存流memoryOutputStream中的數(shù)據(jù)轉(zhuǎn)換到寫入硬盤的流FileOuputStream中新锈，并將FileOuputStream重新賦值currentOuptputStream