Java BufferedReader相關(guān)源碼實(shí)例分析
1、案例代碼
假設(shè)b.txt存儲(chǔ)了abcdegfhijk
public static void main(String[] args) throws IOException { //字符緩沖流 BufferedReader bufferedReader=new BufferedReader(new FileReader (new File("H:\\ioText\\b.txt")),8); //存儲(chǔ)讀取的數(shù)據(jù) char[] charsRead=new char[5]; //讀取數(shù)據(jù) bufferedReader.read(charsRead); //遍歷并輸出charsRead for (char c:charsRead){ System.out.println(c); } }
2、通過(guò)源碼(部分)分析案例
a、第一次讀取
public class BufferedReader extends Reader { private Reader in;//字符流 private char cb[];//緩沖區(qū) private int nChars, nextChar;//nChars緩沖區(qū)可讀字符數(shù),nextChar下一個(gè)字符位置 private static final int INVALIDATED = -2; private static final int UNMARKED = -1; private int markedChar = UNMARKED; private int readAheadLimit = 0; private boolean skipLF = false; private boolean markedSkipLF = false; private static int defaultCharBufferSize = 8192;//緩沖區(qū)默認(rèn)大小 private static int defaultExpectedLineLength = 80; //案例調(diào)用的構(gòu)造方法 public BufferedReader(Reader in, int sz) { //調(diào)用父類構(gòu)造 super(in); //判斷緩沖區(qū)大小是否正常 if (sz <= 0) throw new IllegalArgumentException("Buffer size <= 0"); //用戶傳入的字符流 this.in = in; //給緩沖區(qū)指定空間大?。ò咐付?) cb = new char[sz]; //緩沖區(qū)可讀字符數(shù)和下一個(gè)字符位置初始化為0 nextChar = nChars = 0; } //讀取數(shù)據(jù) public int read(char cbuf[], int off, int len) throws IOException { synchronized (lock) { ensureOpen(); if ((off < 0) || (off > cbuf.length) || (len < 0) || ((off + len) > cbuf.length) || ((off + len) < 0)) { throw new IndexOutOfBoundsException(); } else if (len == 0) { return 0; } //調(diào)用read1方法進(jìn)行讀?。ㄕ嬲x取數(shù)據(jù)的方法是read1方法) int n = read1(cbuf, off, len); if (n <= 0) return n; //將之前沒(méi)處理完的數(shù)據(jù)復(fù)制到自定以數(shù)組charsRead再次調(diào)用read1方法讀取 while ((n < len) && in.ready()) { int n1 = read1(cbuf, off + n, len - n); if (n1 <= 0) break; n += n1; } return n; } } //cbuf用戶自定義數(shù)組(charsRead),off=0,len=5 private int read1(char[] cbuf, int off, int len) throws IOException { if (nextChar >= nChars) {//第一次讀nextChar、nChars都為0,滿足條件 if (len >= cb.length && markedChar <= UNMARKED && !skipLF) { return in.read(cbuf, off, len); } //刷新緩沖區(qū),先往下找到fill方法源碼分析 fill(); } if (nextChar >= nChars) return -1; if (skipLF) { skipLF = false; if (cb[nextChar] == '\n') { nextChar++; if (nextChar >= nChars) fill(); if (nextChar >= nChars) return -1; } } //執(zhí)行完fill方法到這里,(len=5,nChars - nextChar=8-0)->n=5 int n = Math.min(len, nChars - nextChar); //將緩沖區(qū)cb從nextChar開(kāi)始復(fù)制n=5個(gè)字符到自定義數(shù)組 System.arraycopy(cb, nextChar, cbuf, off, n); //nextChar=5 nextChar += n; //n=5 return n; } //刷新緩沖區(qū)方法 private void fill() throws IOException { int dst; if (markedChar <= UNMARKED) {//markedChar初始值為UNMARKED,滿足條件 /* No mark */ dst = 0;//初始化dst } else { /* Marked */ int delta = nextChar - markedChar; if (delta >= readAheadLimit) { /* Gone past read-ahead limit: Invalidate mark */ markedChar = INVALIDATED; readAheadLimit = 0; dst = 0; } else { if (readAheadLimit <= cb.length) { /* Shuffle in the current buffer */ System.arraycopy(cb, markedChar, cb, 0, delta); markedChar = 0; dst = delta; } else { /* Reallocate buffer to accommodate read-ahead limit */ char ncb[] = new char[readAheadLimit]; System.arraycopy(cb, markedChar, ncb, 0, delta); cb = ncb; markedChar = 0; dst = delta; } nextChar = nChars = delta; } } int n; do { //dst=0,cb.length - dst=8-0->n=8 n = in.read(cb, dst, cb.length - dst); } while (n == 0); if (n > 0) {//滿足條件 //nChars=8 nChars = dst + n; //nextChar=0 nextChar = dst; } } }
第一次讀取后charsRead存儲(chǔ)了五個(gè)字符:abcde
b、第二次讀取
//cbuf用戶自定義數(shù)組(charsRead),off=0,len=5 private int read1(char[] cbuf, int off, int len) throws IOException { if (nextChar >= nChars) {//第二次讀nextChar=5、nChars=8,不滿足條件 if (len >= cb.length && markedChar <= UNMARKED && !skipLF) { return in.read(cbuf, off, len); } fill(); } if (nextChar >= nChars) return -1; if (skipLF) { skipLF = false; if (cb[nextChar] == '\n') { nextChar++; if (nextChar >= nChars) fill(); if (nextChar >= nChars) return -1; } } //跳過(guò)if直接到這里,len=5,nChars - nextChar=8-5=3->n=3 int n = Math.min(len, nChars - nextChar); //將緩沖區(qū)cb從nextChar=5開(kāi)始復(fù)制n=3個(gè)字符到自定義數(shù)組 System.arraycopy(cb, nextChar, cbuf, off, n); //nextChar=5+3=8 nextChar += n; //n=8 return n; }
第二次讀取只讀了三個(gè)字符把charsRead五個(gè)字符的前三個(gè)覆蓋:fghde
c、第三次讀取
//cbuf用戶自定義數(shù)組(charsRead),off=0,len=5 private int read1(char[] cbuf, int off, int len) throws IOException { if (nextChar >= nChars) {//第三次讀nextChar=8、nChars=8,滿足條件 if (len >= cb.length && markedChar <= UNMARKED && !skipLF) { return in.read(cbuf, off, len); } //刷新緩沖區(qū),先往下找到fill方法源碼分析 fill(); } if (nextChar >= nChars) return -1; if (skipLF) { skipLF = false; if (cb[nextChar] == '\n') { nextChar++; if (nextChar >= nChars) fill(); if (nextChar >= nChars) return -1; } } //執(zhí)行完fill方法到這里,(len=2,nChars - nextChar=8-0)->n=2 int n = Math.min(len, nChars - nextChar); //將緩沖區(qū)cb從nextChar=0開(kāi)始復(fù)制n=2個(gè)字符到自定義數(shù)組 System.arraycopy(cb, nextChar, cbuf, off, n); //nextChar=5+3=8 nextChar += n; //n=8 return n; } //刷新緩沖區(qū)方法 private void fill() throws IOException { int dst; if (markedChar <= UNMARKED) {//markedChar初始值為UNMARKED,滿足條件 /* No mark */ dst = 0;//初始化dst } else { /* Marked */ int delta = nextChar - markedChar; if (delta >= readAheadLimit) { /* Gone past read-ahead limit: Invalidate mark */ markedChar = INVALIDATED; readAheadLimit = 0; dst = 0; } else { if (readAheadLimit <= cb.length) { /* Shuffle in the current buffer */ System.arraycopy(cb, markedChar, cb, 0, delta); markedChar = 0; dst = delta; } else { /* Reallocate buffer to accommodate read-ahead limit */ char ncb[] = new char[readAheadLimit]; System.arraycopy(cb, markedChar, ncb, 0, delta); cb = ncb; markedChar = 0; dst = delta; } nextChar = nChars = delta; } } int n; do { //dst=0,cb.length - dst=8-0->n=8 n = in.read(cb, dst, cb.length - dst); } while (n == 0); if (n > 0) {//滿足條件 //nChars=8 nChars = dst + n; //nextChar=0 nextChar = dst; } } }
第三次讀取了兩個(gè)字符到charsRead,把最后兩個(gè)字符覆蓋:fghijk
3、源碼執(zhí)行過(guò)程圖解
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