public static void main(String[] args) throws FileNotFoundException {
        PrintStream out = new PrintStream("d:/d.txt");
        System.setOut(out); //改变了标准输出流
        System.out.println(System.out == out);  // 标记1   文件中打印true
        System.out.println("某些数据.");  //这样的话会像上面的文件中写入数据
                            //  问题:   我怎么样才能改回标准输出流呢?   在setOut前没有保存引用.
                            //  疑惑:   System类的out字段是public static final 的, 为什么可以改变呢?
                            //标记1那里居然打印的是true
    }

PrintStream out = new PrintStream("d:/d.txt");
PrintStream systemOut = System.out;//先把System.out取出来保存下来
System.setOut(out);
System.out.println(System.out == out);
System.out.println("某些数据.");
System.setOut(systemOut);//然后再设置回去
System.out.println("变回来了！");

------解决方案--------------------

在System里的initializeSystemClass方法可以看到如何赋予标准输出流
你可以这样
Java code

        FileOutputStream fdOut = new FileOutputStream(FileDescriptor.out);
        System.setOut(new PrintStream(new BufferedOutputStream(fdOut, 128), true));
        System.out.println("某些数据2.");

------解决方案--------------------
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在System里的initializeSystemClass方法可以看到如何赋予标准输出流
你可以这样
Java code

        Normally, final static fields may not be modified. However System.in, System.out, and System.err are final static fields that, for legacy reasons, must be allowed to be changed by the methods System.setIn, System.setOut and System.setErr. We refer to these fields as being write-protected to distinguish them from ordinary final fields.

The compiler needs to treat these fields differently from other final fields. For example, a read of an ordinary final field is "immune" to synchronization: the barrier involved in a lock or volatile read does not have to affect what value is read from a final field. Since the value of write-protected fields may be seen to change, synchronization events should have an effect on them. Therefore, the semantics dictate that these fields be treated as normal fields that cannot be changed by user code, unless that user code is in the System class