JspWriter (Java EE 5)

JSP 页面中的操作和模板数据是使用 JspWriter 对象编写的，该对象由隐式变量 out 引用，而此变量是使用 PageContext 对象中的方法自动初始化的。

此抽象类模仿 java.io.BufferedWriter 和 java.io.PrintWriter 类中的一些功能，但又不同于这些类，因为它从 print 方法抛出 java.io.IOException，而 PrintWriter 不会这样做。

缓冲

初始 JspWriter 对象与 ServletResponse 的 PrintWriter 对象关联，关联方式取决于页面是否被缓冲。如果页面未被缓冲，则写入此 JspWriter 对象的输出将直接写入 PrintWriter，如有必要，将通过对响应对象调用 getWriter() 方法创建 PrintWriter。但是，如果页面被缓冲，则在刷新缓冲区之前不会创建 PrintWriter 对象，并且 setContentType() 之类的操作是合法的。因为这种灵活性显着简化了编程，所以默认情况下将对 JSP 页面进行缓冲。

缓冲会带来超出缓冲区时该如何处理的问题。可以采用两种方法：

• 超出缓冲区不是致命错误；在超出缓冲区时，只需刷新输出即可。
• 超出缓冲区是致命错误；在超出缓冲区时，引发一个异常。

两种方法都有效，并且在 JSP 技术中都得到支持。页面的行为由 autoFlush 属性控制，该属性的默认值为 true。通常，需要确保已将正确且完整的数据发送到其客户端的 JSP 页面可能想将 autoFlush 设置为 false，比如客户端是应用程序自身的情况。另一方面，发送有意义的数据（即使只构造了一部分时）的 JSP 页面可能想将 autoFlush 设置为 true；比如发送要通过浏览器立即显示的数据时。每个应用程序都需要考虑自己的特定需求。

一个可以考虑的选择是不设置缓冲区大小界限；但是，这又带来了计算不受控制的弊病，将消耗数量无法估计的资源。

JSP 实现类的 "out" 隐式变量就属于这种类型。如果页面指令选择 autoflush="true"，那么在导致溢出情况时，或者在当前操作是在没有进行刷新的情况下执行的时候，此类上的所有 I/O 操作都将自动刷新缓冲区的内容。如果 autoflush="false"，那么在执行当前操作将导致缓冲区溢出的情况下，此类上的所有 I/O 操作都将抛出 IOException。

The actions and template data in a JSP page is written using the JspWriter object that is referenced by the implicit variable out which is initialized automatically using methods in the PageContext object.

This abstract class emulates some of the functionality found in the java.io.BufferedWriter and java.io.PrintWriter classes, however it differs in that it throws java.io.IOException from the print methods while PrintWriter does not.

Buffering

The initial JspWriter object is associated with the PrintWriter object of the ServletResponse in a way that depends on whether the page is or is not buffered. If the page is not buffered, output written to this JspWriter object will be written through to the PrintWriter directly, which will be created if necessary by invoking the getWriter() method on the response object. But if the page is buffered, the PrintWriter object will not be created until the buffer is flushed and operations like setContentType() are legal. Since this flexibility simplifies programming substantially, buffering is the default for JSP pages.

Buffering raises the issue of what to do when the buffer is exceeded. Two approaches can be taken:

• Exceeding the buffer is not a fatal error; when the buffer is exceeded, just flush the output.
• Exceeding the buffer is a fatal error; when the buffer is exceeded, raise an exception.

Both approaches are valid, and thus both are supported in the JSP technology. The behavior of a page is controlled by the autoFlush attribute, which defaults to true. In general, JSP pages that need to be sure that correct and complete data has been sent to their client may want to set autoFlush to false, with a typical case being that where the client is an application itself. On the other hand, JSP pages that send data that is meaningful even when partially constructed may want to set autoFlush to true; such as when the data is sent for immediate display through a browser. Each application will need to consider their specific needs.

An alternative considered was to make the buffer size unbounded; but, this had the disadvantage that runaway computations would consume an unbounded amount of resources.

The "out" implicit variable of a JSP implementation class is of this type. If the page directive selects autoflush="true" then all the I/O operations on this class shall automatically flush the contents of the buffer if an overflow condition would result if the current operation were performed without a flush. If autoflush="false" then all the I/O operations on this class shall throw an IOException if performing the current operation would result in a buffer overflow condition.

See Also:

Writer, BufferedWriter, PrintWriter

英文文档：

NO_BUFFER

public static final int NO_BUFFER

Constant indicating that the Writer is not buffering output.

See Also:

Constant Field Values

英文文档：

DEFAULT_BUFFER

public static final int DEFAULT_BUFFER

Constant indicating that the Writer is buffered and is using the implementation default buffer size.

See Also:

Constant Field Values

英文文档：

UNBOUNDED_BUFFER

public static final int UNBOUNDED_BUFFER

Constant indicating that the Writer is buffered and is unbounded; this is used in BodyContent.

See Also:

Constant Field Values

英文文档：

bufferSize

protected int bufferSize

The size of the buffer used by the JspWriter.

英文文档：

autoFlush

protected boolean autoFlush

Whether the JspWriter is autoflushing.

protected JspWriter(int bufferSize, boolean autoFlush)
受保护的构造方法。

bufferSize	将由 JspWriter 使用的缓冲区的大小
autoFlush	JspWriter 是否应自动刷新

英文文档：

JspWriter

protected JspWriter(int bufferSize,
                    boolean autoFlush)

Protected constructor.

Parameters:

bufferSize - the size of the buffer to be used by the JspWriter

autoFlush - whether the JspWriter should be autoflushing

abstract public void newLine() throws java.io.IOException
写入一个行分隔符。行分隔符字符串由系统属性 line.separator 定义，并且不一定是单个换行 ('\n') 字符。

Throws

java.io.IOException: 如果发生 I/O 错误

英文文档：

newLine

public abstract void newLine()
                      throws IOException

Write a line separator. The line separator string is defined by the system property line.separator, and is not necessarily a single newline ('\n') character.

Throws:

IOException - If an I/O error occurs

abstract public void print(boolean b) throws java.io.IOException
打印 boolean 值。将 valueOf(boolean) 生成的字符串写入 JspWriter 的缓冲区，如果没有使用缓冲区，则直接将该字符串写入底层 Writer。

b	要打印的 boolean
Throws	java.io.IOException: 如果写入时发生错误

英文文档：

print

public abstract void print(boolean b)
                    throws IOException

public abstract void print(char c)
                    throws IOException

public abstract void print(int i)
                    throws IOException

public abstract void print(long l)
                    throws IOException

public abstract void print(float f)
                    throws IOException

public abstract void print(double d)
                    throws IOException

public abstract void print(char[] s)
                    throws IOException

public abstract void print(String s)
                    throws IOException

public abstract void print(Object obj)
                    throws IOException

public abstract void println()
                      throws IOException

public abstract void println(boolean x)
                      throws IOException

public abstract void println(char x)
                      throws IOException

public abstract void println(int x)
                      throws IOException

public abstract void println(long x)
                      throws IOException

public abstract void println(float x)
                      throws IOException

public abstract void println(double x)
                      throws IOException

public abstract void println(char[] x)
                      throws IOException

public abstract void println(String x)
                      throws IOException

public abstract void println(Object x)
                      throws IOException

public abstract void clear()
                    throws IOException

public abstract void clearBuffer()
                          throws IOException

public abstract void flush()
                    throws IOException

public abstract void close()
                    throws IOException

public int getBufferSize()

public abstract int getRemaining()

public boolean isAutoFlush()