Java System.out未被重定向

Question

我有一個捕獲所有System.out輸出的簡單任務; 然而，我失敗了：

public class Main {
    ScriptEngine engine = new ScriptEngineManager().getEngineByName("js");
    ArrayList<String> history = new ArrayList<>();
    StringBuilder output = new StringBuilder();
    StringBuilder input = new StringBuilder();
    JPanel panel = new JPanel();
    JTextArea txt = new JTextArea();
    String PROMPT = "> ";
    Runnable show = new Runnable() {
            public void run() {
                txt.setText("");
                for (String line: history) txt.append(line + "\n");
                txt.append(PROMPT);
                txt.append(output.toString());
                txt.setCaretPosition(txt.getText().length());
            }
        };
    PrintStream os = new PrintStream(new OutputStream() {
            @Override
            public void write(int b) throws IOException {
                if (b == 13) {
                    history.add(input.toString());
                    input = new StringBuilder();
                    SwingUtilities.invokeLater(show);
                } else input.append((char)b);
            }
        });
    void init(String title) {
        panel.setLayout(new BorderLayout());
        txt.setLineWrap(true);
        txt.setFont(new Font("Courier", Font.PLAIN, 16));
        txt.setBackground(Color.BLACK);
        txt.setForeground(Color.GREEN);
        txt.setCaretColor(Color.GREEN);
        txt.setEditable(false);
        txt.setText(PROMPT);
        txt.addKeyListener(new KeyListener() {
            @Override
            public void keyTyped(KeyEvent e) {
                if (e.getKeyChar() == '\n') {
                    System.setOut(os);
                    System.setErr(os);
                    String result = output.toString();
                    history.add(PROMPT + result);
                    try {
                        engine.eval(result);
                    } catch (ScriptException ex) {
                        history.add(ex.toString());
                    }
                    output = new StringBuilder();
                } else {
                    output.append(e.getKeyChar());
                }
                SwingUtilities.invokeLater(show);
            }
            @Override
            public void keyPressed(KeyEvent e) {

            }
            @Override
            public void keyReleased(KeyEvent e) {

            }
        });
        panel.add(txt, BorderLayout.CENTER);
        JFrame frame = new JFrame(title);
        frame.setSize(650, 425);
        frame.setContentPane(panel);
        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }
    public static void main(String args[]) {
        new Main().init("javascript");
    }
}

我應該注意我正在尋找的輸出來自：

new ScriptEngineManager().getEngineByName("js").eval("print(\"test\");");

除了普通的STDOUT又稱System.out之外，輸出不會發送到任何地方。 為什么？

Answer 1

ScriptEngine輸出重定向：

要重定向腳本的輸出，首先必須獲取其ScriptContext實例，該實例具有名為setWriter（Writer）的方法。 這設置了ScriptEngine的輸出。

為此你首先需要一個作家。 這可以像這樣簡單：

public class CaptureWriter extends Writer
{

    private StringBuilder m_build;

    public CaptureWriter(StringBuilder build)
    {
        m_build = build;
    }

    @Override
    public void write(char[] cbuf, int off, int len) throws IOException
    {
        m_build.insert(m_build.length(), cbuf, off, len);
    }

    @Override
    public void flush() throws IOException
    {
    }

    @Override
    public void close() throws IOException
    {
    }

}

這會將所有輸入寫入StringBuilder。

然后將其注冊到ScriptContext中

ScriptEngine engine = new ScriptEngineManager().getEngineByName("js");

engine.getContext().setWriter(new CaptureWriter(m_mess));

運行這個簡單的程序時：

StringBuilder build = new StringBuilder();

    ScriptEngine engine = new ScriptEngineManager().getEngineByName("js");

    engine.getContext().setWriter(new CaptureWriter(build));

    try
    {
        engine.eval("print(\"Hello\")");
        engine.eval("print(\"World\")");
        engine.eval("print(\"You\")");
        engine.eval("print(\"There\")");
    } catch(ScriptException e)
    {
        e.printStackTrace();
    }

    System.out.println(build);

輸出是腳本的緩沖輸出：

Hello
World
You
There

您當然可以將任何內容掛鈎到Writer實現的write方法中，這只是一個示例。

System.out重定向：

要捕獲System.out輸出，您必須創建自己的PrintStream實現，它可以非常簡單，如：

public class Capture extends PrintStream
{

    private StringBuilder m_build;

    public Capture(OutputStream out, StringBuilder build)
    {
        super(out);

        m_build = build;
    }

    public void println(String s)
    {
        super.println(s);

        m_build.append(s + "\n");
    }

}

然后你需要在運行時注冊它：

StringBuilder build = new StringBuilder();

System.setOut(new Capture(System.out, build));

然后每次調用System.out.println時，它都會像重寫PrintStream那樣工作，並且消息會寫入提供的StringBuilder。

要捕獲其他消息，您需要覆蓋其他方法。

Answer 2

您可以捕獲System.out提供您控制的PrintStream ...嘗試這樣的事情

ByteArrayOutputStream baos = new ByteArrayOutputStream();
PrintStream ps = new PrintStream(baos);
PrintStream old = System.out;
System.setOut(ps);
System.out.println("This will be captured :P");
System.out.flush();
System.setOut(old);
System.out.println("He is what was captured : " + baos.toString());