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print打印字符串之謎 博客分類： java

string 中維護了一個char數組的value值，打印的時候也就是打印的這個數組的值。但是print(str)參數為string類型

?

看print的源碼

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/*** Prints a String and then terminate the line. This method behaves as* though it invokes <code>{@link #print(String)}</code> and then* <code>{@link #println()}</code>.** @param x The <code>String</code> to be printed.*/public void println(String x) {synchronized (this) {print(x);newLine();}}

? ? public void print(String s) {

if (s == null) {s = "null";}write(s);}

? ? private void write(String s) {

try {synchronized (this) {ensureOpen();textOut.write(s);//java.io.BufferedWriter.write(s)textOut.flushBuffer();charOut.flushBuffer();//java.io.OutputStreamWriter.flushBuffer();if (autoFlush && (s.indexOf('\n') >= 0))out.flush();}}

?//在看看write方法調用了getChars方法，這是輸出string，char數組值的關鍵

public void write(String s, int off, int len) throws IOException {synchronized (lock) {ensureOpen();int b = off, t = off + len;while (b < t) {int d = min(nChars - nextChar, t - b);s.getChars(b, b + d, cb, nextChar);b += d;nextChar += d;if (nextChar >= nChars)flushBuffer();}}}

? ?//最終調用了printStream方法的write方法

public void write(byte buf[], int off, int len) {try {synchronized (this) {ensureOpen();out.write(buf, off, len);if (autoFlush)out.flush();//關鍵在這一步調用了java.io.BufferedOutputStream的flushBuffer方法}}catch (InterruptedIOException x) {Thread.currentThread().interrupt();}

??java.io.BufferedOutputStream的flushBuffer方法

/** Flush the internal buffer */最終調用了java.io.FileOutputStream..write(buf, 0, count);方法private void flushBuffer() throws IOException {if (count > 0) {out.write(buf, 0, count);//java.io.FileOutputStream..write(buf, 0, count);count = 0;}}

? ? //?java.io.FileOutputStream..write最終調用了writeBytes方法

public void write(byte b[], int off, int len) throws IOException {writeBytes(b, off, len);}

??//writeBytesa方法是本地方法最終以byte數組輸出

private native void writeBytes(byte b[], int off, int len) throws IOException;

? ?jdK native code

??JNIEXPORT void JNICALL

Java_java_io_FileOutputStream_writeBytes(JNIEnv *env,jobject this, jbyteArray bytes, jint off, jint len) {jboolean append = (*env)->GetBooleanField(env, this, fos_append);FD fd = GET_FD(this, fos_fd);if (fd == -1) {JNU_ThrowIOException(env, "Stream Closed");return;}if (append == JNI_TRUE) {if (IO_Lseek(fd, 0L, SEEK_END) == -1) {JNU_ThrowIOExceptionWithLastError(env, "Append failed");}}writeBytes(env, this, bytes, off, len, fos_fd); }

轉載于:https://my.oschina.net/xiaominmin/blog/1598168

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