print打印字符串之谜

string 中维护了一个char数组的value值，打印的时候也就是打印的这个数组的值。但是print(str)参数为string类型

 

看print的源码

 

   /**
     * 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方法调用了getChars方法，这是输出string，char数组值的关键

 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();
	    }
	}
    }

   //最终调用了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();//关键在这一步调用了java.io.BufferedOutputStream的flushBuffer方法
	    }
	}
	catch (InterruptedIOException x) {
	    Thread.currentThread().interrupt();
	}
	

  java.io.BufferedOutputStream的flushBuffer方法

  /** Flush the internal buffer */最终调用了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最终调用了writeBytes方法

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

  //writeBytesa方法是本地方法最终以byte数组输出

 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);
}