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);
}