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Java IO 之 FileInputStream & FileOutputStream 源码分析

  • 作者: 十个丶钟
  • 来源: 51数据库
  • 2020-08-14

一、引子

文件，作为常见的数据源。关于操作文件的字节流就是 —?FileInputStream?&?FileOutputStream。它们是Basic IO字节流中重要的实现类。

二、FileInputStream源码分析

FileInputStream源码如下：

/**
?* FileInputStream 从文件系统的文件中获取输入字节流。文件取决于主机系统。
?*? 比如读取图片等的原始字节流。如果读取字符流，考虑使用 FiLeReader。
?*/
public class SFileInputStream extends InputStream
{
????/* 文件描述符类---此处用于打开文件的句柄 */
????private final FileDescriptor fd;

????/* 引用文件的路径 */
????private final String path;

????/* 文件通道，NIO部分 */
????private FileChannel channel = null;

????private final Object closeLock = new Object();
????private volatile boolean closed = false;

????private static final ThreadLocal<Boolean> runningFinalize =
????????new ThreadLocal<>();

????private static boolean isRunningFinalize() {
????????Boolean val;
????????if ((val = runningFinalize.get()) != null)
????????????return val.booleanValue();
????????return false;
????}

????/* 通过文件路径名来创建FileInputStream */
????public FileInputStream(String name) throws FileNotFoundException {
????????this(name != null ? new File(name) : null);
????}

????/* 通过文件来创建FileInputStream */
????public FileInputStream(File file) throws FileNotFoundException {
????????String name = (file != null ? file.getPath() : null);
????????SecurityManager security = System.getSecurityManager();
????????if (security != null) {
????????????security.checkRead(name);
????????}
????????if (name == null) {
????????????throw new NullPointerException();
????????}
????????if (file.isInvalid()) {
????????????throw new FileNotFoundException("Invalid file path");
????????}
????????fd = new FileDescriptor();
????????fd.incrementAndGetUseCount();
????????this.path = name;
????????open(name);
????}

????/* 通过文件描述符类来创建FileInputStream */
????public FileInputStream(FileDescriptor fdObj) {
????????SecurityManager security = System.getSecurityManager();
????????if (fdObj == null) {
????????????throw new NullPointerException();
????????}
????????if (security != null) {
????????????security.checkRead(fdObj);
????????}
????????fd = fdObj;
????????path = null;
????????fd.incrementAndGetUseCount();
????}

????/* 打开文件，为了下一步读取文件内容。native方法 */
????private native void open(String name) throws FileNotFoundException;

????/* 从此输入流中读取一个数据字节 */
????public int read() throws IOException {
????????Object traceContext = IoTrace.fileReadBegin(path);
????????int b = 0;
????????try {
????????????b = read0();
????????} finally {
????????????IoTrace.fileReadEnd(traceContext, b == -1 ? 0 : 1);
????????}
????????return b;
????}

????/* 从此输入流中读取一个数据字节。native方法 */
????private native int read0() throws IOException;

????/* 从此输入流中读取多个字节到byte数组中。native方法 */
????private native int readBytes(byte b[], int off, int len) throws IOException;

????/* 从此输入流中读取多个字节到byte数组中。 */
????public int read(byte b[]) throws IOException {
????????Object traceContext = IoTrace.fileReadBegin(path);
????????int bytesRead = 0;
????????try {
????????????bytesRead = readBytes(b, 0, b.length);
????????} finally {
????????????IoTrace.fileReadEnd(traceContext, bytesRead == -1 ? 0 : bytesRead);
????????}
????????return bytesRead;
????}

????/* 从此输入流中读取最多len个字节到byte数组中。 */
????public int read(byte b[], int off, int len) throws IOException {
????????Object traceContext = IoTrace.fileReadBegin(path);
????????int bytesRead = 0;
????????try {
????????????bytesRead = readBytes(b, off, len);
????????} finally {
????????????IoTrace.fileReadEnd(traceContext, bytesRead == -1 ? 0 : bytesRead);
????????}
????????return bytesRead;
????}

????public native long skip(long n) throws IOException;

????/* 返回下一次对此输入流调用的方法可以不受阻塞地从此输入流读取（或跳过）的估计剩余字节数。 */
????public native int available() throws IOException;

????/* 关闭此文件输入流并释放与此流有关的所有系统资源。 */
????public void close() throws IOException {
????????synchronized (closeLock) {
????????????if (closed) {
????????????????return;
????????????}
????????????closed = true;
????????}
????????if (channel != null) {
???????????fd.decrementAndGetUseCount();
???????????channel.close();
????????}

????????int useCount = fd.decrementAndGetUseCount();

????????if ((useCount <= 0) || !isRunningFinalize()) {
????????????close0();
????????}
????}

????public final FileDescriptor getFD() throws IOException {
????????if (fd != null) return fd;
????????throw new IOException();
????}

????/* 获取此文件输入流的唯一FileChannel对象 */
????public FileChannel getChannel() {
????????synchronized (this) {
????????????if (channel == null) {
????????????????channel = FileChannelImpl.open(fd, path, true, false, this);
????????????????fd.incrementAndGetUseCount();
????????????}
????????????return channel;
????????}
????}

????private static native void initIDs();

????private native void close0() throws IOException;

????static {
????????initIDs();
????}

????protected void finalize() throws IOException {
????????if ((fd != null) &&? (fd != FileDescriptor.in)) {
????????????runningFinalize.set(Boolean.TRUE);
????????????try {
????????????????close();
????????????} finally {
????????????????runningFinalize.set(Boolean.FALSE);
????????????}
????????}
????}
}

1. 三个核心方法

三个核心方法，也就是Override（重写）了抽象类InputStream的read方法。

int read()?方法，即

public int read() throws IOException

代码实现中很简单，一个try中调用本地native的read0()方法，直接从文件输入流中读取一个字节。IoTrace.fileReadEnd(),字面意思是防止文件没有关闭读的通道，导致读文件失败，一直开着读的通道，会造成内存泄露。

int read(byte b[])?方法，即

public int read(byte b[]) throws IOException

代码实现也是比较简单的，也是一个try中调用本地native的readBytes()方法，直接从文件输入流中读取最多b.length个字节到byte数组b中。

int read(byte b[], int off, int len)?方法，即

public int read(byte b[], int off, int len) throws IOException

代码实现和 int read(byte b[])方法 一样，直接从文件输入流中读取最多len个字节到byte数组b中。

可是这里有个问答：

Q： 为什么 int read(byte b[]) 方法需要自己独立实现呢？ 直接调用 int read(byte b[], int off, int len) 方法，即read(b , 0 , b.length)，等价于read(b)？

A：待完善，希望路过大神回答。。。。向下兼容？？ Finally？？

2. 值得一提的native方法

上面核心方法中为什么实现简单，因为工作量都在native方法里面，即JVM里面实现了。native倒是不少一一列举吧：

native void open(String name) // 打开文件，为了下一步读取文件内容

native int read0() // 从文件输入流中读取一个字节

native int readBytes(byte b[], int off, int len) // 从文件输入流中读取，从off句柄开始的len个字节，并存储至b字节数组内。

native void close0() // 关闭该文件输入流及涉及的资源，比如说如果该文件输入流的FileChannel对被获取后，需要对FileChannel进行close。

其他还有值得一提的就是，在jdk1.4中，新增了NIO包，优化了一些IO处理的速度，所以在FileInputStream和FileOutputStream中新增了FileChannel?getChannel()的方法。即获取与该文件输入流相关的 java.nio.channels.FileChannel对象。

三、FileOutputStream 源码分析

FileOutputStream 源码如下：

/**
?* 文件输入流是用于将数据写入文件或者文件描述符类
?*? 比如写入图片等的原始字节流。如果写入字符流，考虑使用 FiLeWriter。
?*/
public class SFileOutputStream extends OutputStream
{
????/* 文件描述符类---此处用于打开文件的句柄 */
????private final FileDescriptor fd;

????/* 引用文件的路径 */
????private final String path;

????/* 如果为 true，则将字节写入文件末尾处，而不是写入文件开始处 */
????private final boolean append;

????/* 关联的FileChannel类，懒加载 */
????private FileChannel channel;

????private final Object closeLock = new Object();
????private volatile boolean closed = false;
????private static final ThreadLocal<Boolean> runningFinalize =
????????new ThreadLocal<>();

????private static boolean isRunningFinalize() {
????????Boolean val;
????????if ((val = runningFinalize.get()) != null)
????????????return val.booleanValue();
????????return false;
????}

????/* 通过文件名创建文件输入流 */
????public FileOutputStream(String name) throws FileNotFoundException {
????????this(name != null ? new File(name) : null, false);
????}

????/* 通过文件名创建文件输入流，并确定文件写入起始处模式 */
????public FileOutputStream(String name, boolean append)
????????throws FileNotFoundException
????{
????????this(name != null ? new File(name) : null, append);
????}

????/* 通过文件创建文件输入流，默认写入文件的开始处 */
????public FileOutputStream(File file) throws FileNotFoundException {
????????this(file, false);
????}

????/* 通过文件创建文件输入流，并确定文件写入起始处? */
????public FileOutputStream(File file, boolean append)
????????throws FileNotFoundException
????{
????????String name = (file != null ? file.getPath() : null);
????????SecurityManager security = System.getSecurityManager();
????????if (security != null) {
????????????security.checkWrite(name);
????????}
????????if (name == null) {
????????????throw new NullPointerException();
????????}
????????if (file.isInvalid()) {
????????????throw new FileNotFoundException("Invalid file path");
????????}
????????this.fd = new FileDescriptor();
????????this.append = append;
????????this.path = name;
????????fd.incrementAndGetUseCount();
????????open(name, append);
????}

????/* 通过文件描述符类创建文件输入流 */
????public FileOutputStream(FileDescriptor fdObj) {
????????SecurityManager security = System.getSecurityManager();
????????if (fdObj == null) {
????????????throw new NullPointerException();
????????}
????????if (security != null) {
????????????security.checkWrite(fdObj);
????????}
????????this.fd = fdObj;
????????this.path = null;
????????this.append = false;

????????fd.incrementAndGetUseCount();
????}

????/* 打开文件，并确定文件写入起始处模式 */
????private native void open(String name, boolean append)
????????throws FileNotFoundException;

????/* 将指定的字节b写入到该文件输入流，并指定文件写入起始处模式 */
????private native void write(int b, boolean append) throws IOException;

????/* 将指定的字节b写入到该文件输入流 */
????public void write(int b) throws IOException {
????????Object traceContext = IoTrace.fileWriteBegin(path);
????????int bytesWritten = 0;
????????try {
????????????write(b, append);
????????????bytesWritten = 1;
????????} finally {
????????????IoTrace.fileWriteEnd(traceContext, bytesWritten);
????????}
????}

????/* 将指定的字节数组写入该文件输入流，并指定文件写入起始处模式 */
????private native void writeBytes(byte b[], int off, int len, boolean append)
????????throws IOException;

????/* 将指定的字节数组b写入该文件输入流 */
????public void write(byte b[]) throws IOException {
????????Object traceContext = IoTrace.fileWriteBegin(path);
????????int bytesWritten = 0;
????????try {
????????????writeBytes(b, 0, b.length, append);
????????????bytesWritten = b.length;
????????} finally {
????????????IoTrace.fileWriteEnd(traceContext, bytesWritten);
????????}
????}

????/* 将指定len长度的字节数组b写入该文件输入流 */
????public void write(byte b[], int off, int len) throws IOException {
????????Object traceContext = IoTrace.fileWriteBegin(path);
????????int bytesWritten = 0;
????????try {
????????????writeBytes(b, off, len, append);
????????????bytesWritten = len;
????????} finally {
????????????IoTrace.fileWriteEnd(traceContext, bytesWritten);
????????}
????}

????/* 关闭此文件输出流并释放与此流有关的所有系统资源 */
????public void close() throws IOException {
????????synchronized (closeLock) {
????????????if (closed) {
????????????????return;
????????????}
????????????closed = true;
????????}

????????if (channel != null) {
????????????fd.decrementAndGetUseCount();
????????????channel.close();
????????}

????????int useCount = fd.decrementAndGetUseCount();

????????if ((useCount <= 0) || !isRunningFinalize()) {
????????????close0();
????????}
????}

?????public final FileDescriptor getFD()? throws IOException {
????????if (fd != null) return fd;
????????throw new IOException();
?????}

????public FileChannel getChannel() {
????????synchronized (this) {
????????????if (channel == null) {
????????????????channel = FileChannelImpl.open(fd, path, false, true, append, this);

????????????????fd.incrementAndGetUseCount();
????????????}
????????????return channel;
????????}
????}

????protected void finalize() throws IOException {
????????if (fd != null) {
????????????if (fd == FileDescriptor.out || fd == FileDescriptor.err) {
????????????????flush();
????????????} else {

????????????????runningFinalize.set(Boolean.TRUE);
????????????????try {
????????????????????close();
????????????????} finally {
????????????????????runningFinalize.set(Boolean.FALSE);
????????????????}
????????????}
????????}
????}

????private native void close0() throws IOException;

????private static native void initIDs();

????static {
????????initIDs();
????}

}

1. 三个核心方法

三个核心方法，也就是Override（重写）了抽象类OutputStream的write方法。

void write(int b)?方法，即

public void write(int b) throws IOException

代码实现中很简单，一个try中调用本地native的write()方法，直接将指定的字节b写入文件输出流。IoTrace.fileReadEnd()的意思和上面FileInputStream意思一致。

void write(byte b[])?方法，即

public void write(byte b[]) throws IOException

代码实现也是比较简单的，也是一个try中调用本地native的writeBytes()方法，直接将指定的字节数组写入该文件输入流。

void write(byte b[], int off, int len)?方法，即

public void write(byte b[], int off, int len) throws IOException

代码实现和?void write(byte b[])?方法 一样，直接将指定的字节数组写入该文件输入流。

2. 值得一提的native方法

上面核心方法中为什么实现简单，因为工作量都在native方法里面，即JVM里面实现了。native倒是不少一一列举吧：

native void open(String name) // 打开文件，为了下一步读取文件内容

native void write(int b, boolean append) // 直接将指定的字节b写入文件输出流

native native void writeBytes(byte b[], int off, int len, boolean append) // 直接将指定的字节数组写入该文件输入流。

native void close0() // 关闭该文件输入流及涉及的资源，比如说如果该文件输入流的FileChannel对被获取后，需要对FileChannel进行close。

相似之处：

其实到这里，该想一想。两个源码实现很相似，而且native方法也很相似。其实不能说“相似”，应该以“对应”来概括它们。

它们是一组，是一根吸管的两个孔的关系：“一个Input一个Output”。

四、使用案例

下面先看代码：

package org.javacore.io;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;

/*
?* Copyright [2015] [Jeff Lee]
?*
?* Licensed under the Apache License, Version 2.0 (the "License");
?* you may not use this file except in compliance with the License.
?* You may obtain a copy of the License at
?*
?*?? http://www.apache.org/licenses/LICENSE-2.0
?*
?* Unless required by applicable law or agreed to in writing, software
?* distributed under the License is distributed on an "AS IS" BASIS,
?* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
?* See the License for the specific language governing permissions and
?* limitations under the License.
?*/

/**
?* @author Jeff Lee
?* @since 2015-10-8 20:06:03
?* FileInputStream&FileOutputStream使用案例
?*/
public class FileIOStreamT {
????private static final String thisFilePath =
????????????"src" + File.separator +
????????????"org" + File.separator +
????????????"javacore" + File.separator +
????????????"io" + File.separator +
????????????"FileIOStreamT.java";
????public static void main(String[] args) throws IOException {
????????// 创建文件输入流
????????FileInputStream fileInputStream = new FileInputStream(thisFilePath);
????????// 创建文件输出流
????????FileOutputStream fileOutputStream =? new FileOutputStream("data.txt");

????????// 创建流的最大字节数组
????????byte[] inOutBytes = new byte[fileInputStream.available()];
????????// 将文件输入流读取，保存至inOutBytes数组
????????fileInputStream.read(inOutBytes);
????????// 将inOutBytes数组，写出到data.txt文件中
????????fileOutputStream.write(inOutBytes);

????????fileOutputStream.close();
????????fileInputStream.close();
????}
}

运行后，会发现根目录中出现了一个“data.txt”文件，内容为上面的代码。

1. 简单地分析下源码：

1、创建了FileInputStream，读取该代码文件为文件输入流。

2、创建了FileOutputStream，作为文件输出流，输出至data.txt文件。

3、针对流的字节数组，一个 read ，一个write，完成读取和写入。

4、关闭流

2. 代码调用的流程如图所示：

3. 代码虽简单，但是有点小问题：

FileInputStream.available() 是返回流中的估计剩余字节数。所以一般不会用此方法。

一般做法，比如创建一个 byte数组，大小1K。然后read至其返回值不为-1，一直读取即可。边读边写。

五、思考与小结

FileInputStream & FileOutputStream 是一对来自 InputStream和OutputStream的实现类。用于本地文件读写（二进制格式按顺序读写）。

本文小结：

1、FileInputStream 源码分析

2、FileOutputStream 资源分析

3、FileInputStream & FileOutputStream 使用案例

4、其源码调用过程

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