abstract方法可以直接调用？

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先看一段代码：

import java.io.IOException;public class RuntimeTest {public static void main(String[] args) throws IOException, InterruptedException{Process p=Runtime.getRuntime().exec("notepad.exe");p.waitFor();System.out.println("end!");}

查看文档，发现Process类的waitFor()方法是一个abstract方法。可是为什么可以直接调用啊？？？

恩，直觉上想，应该是下面这行代码，实际返回的是Process类的一个子类，在子类中重写了waitFor()方法。

Runtime.getRuntime().exec("notepad.exe");

那实际情况呢？我们查看下源码JDK1.5：

Runtime的源码，重载了好几个exec方法，但所有的exec方法都是调用下面这个方法来实现的。

    public Process exec(String[] cmdarray, String[] envp, File dir)        throws IOException {        return new ProcessBuilder(cmdarray)            .environment(envp)            .directory(dir)            .start();    }

那我们再来查看ProcessBuilder的start方法源码：

  public Process start() throws IOException {        // Must convert to array first -- a malicious user-supplied        // list might try to circumvent the security check.        String[] cmdarray = command.toArray(new String[command.size()]);        cmdarray = cmdarray.clone();        for (String arg : cmdarray)            if (arg == null)                throw new NullPointerException();        // Throws IndexOutOfBoundsException if command is empty        String prog = cmdarray[0];        SecurityManager security = System.getSecurityManager();        if (security != null) {            security.checkExec(prog);        }        String dir = directory == null ? null : directory.toString();        try {            return ProcessImpl.start(cmdarray,                                     environment,                                     dir,                                     redirects,                                     redirectErrorStream);        } catch (IOException | IllegalArgumentException e) {            String exceptionInfo = ": " + e.getMessage();            Throwable cause = e;            if ((e instanceof IOException) && security != null) {                // Can not disclose the fail reason for read-protected files.                try {                    security.checkRead(prog);                } catch (AccessControlException ace) {                    exceptionInfo = "";                    cause = ace;                }            }            // It's much easier for us to create a high-quality error            // message than the low-level C code which found the problem.            throw new IOException(                "Cannot run program \"" + prog + "\""                + (dir == null ? "" : " (in directory \"" + dir + "\")")                + exceptionInfo,                cause);        }    }

可以看到，返回的是：ProcessImpl.start(cmdarray,environment,dir, redirects, redirectErrorStream);，那就再查看ProcessImpl类的源码。

package java.lang;import java.io.IOException;import java.io.File;import java.io.InputStream;import java.io.OutputStream;import java.io.FileInputStream;import java.io.FileOutputStream;import java.io.FileDescriptor;import java.io.BufferedInputStream;import java.io.BufferedOutputStream;import java.lang.ProcessBuilder.Redirect;import java.security.AccessController;import java.security.PrivilegedAction;import java.util.ArrayList;import java.util.regex.Matcher;import java.util.regex.Pattern;/* This class is for the exclusive use of ProcessBuilder.start() to * create new processes. * * @author Martin Buchholz * @since   1.5 */final class ProcessImpl extends Process {<pre name="code" class="java">/**中间部分代码省略*/

// System-dependent portion of ProcessBuilder.start() static Process start(String cmdarray[], java.util.Map<String,String> environment, String dir, ProcessBuilder.Redirect[] redirects, boolean redirectErrorStream) throws IOException { String envblock = ProcessEnvironment.toEnvironmentBlock(environment); FileInputStream f0 = null; FileOutputStream f1 = null; FileOutputStream f2 = null; try { long[] stdHandles; if (redirects == null) { stdHandles = new long[] { -1L, -1L, -1L }; } else { stdHandles = new long[3]; if (redirects[0] == Redirect.PIPE) stdHandles[0] = -1L; else if (redirects[0] == Redirect.INHERIT) stdHandles[0] = fdAccess.getHandle(FileDescriptor.in); else { f0 = new FileInputStream(redirects[0].file()); stdHandles[0] = fdAccess.getHandle(f0.getFD()); } if (redirects[1] == Redirect.PIPE) stdHandles[1] = -1L; else if (redirects[1] == Redirect.INHERIT) stdHandles[1] = fdAccess.getHandle(FileDescriptor.out); else { f1 = newFileOutputStream(redirects[1].file(), redirects[1].append()); stdHandles[1] = fdAccess.getHandle(f1.getFD()); } if (redirects[2] == Redirect.PIPE) stdHandles[2] = -1L; else if (redirects[2] == Redirect.INHERIT) stdHandles[2] = fdAccess.getHandle(FileDescriptor.err); else { f2 = newFileOutputStream(redirects[2].file(), redirects[2].append()); stdHandles[2] = fdAccess.getHandle(f2.getFD()); } } return new ProcessImpl(cmdarray, envblock, dir, stdHandles, redirectErrorStream); } finally { // In theory, close() can throw IOException // (although it is rather unlikely to happen here) try { if (f0 != null) f0.close(); } finally { try { if (f1 != null) f1.close(); } finally { if (f2 != null) f2.close(); } } }
/*
*中间部分代码省略
*/
public int waitFor() throws InterruptedException { waitForInterruptibly(handle); if (Thread.interrupted()) throw new InterruptedException(); return exitValue(); } private static native void waitForInterruptibly(long handle);}

可以看出，ProcessImpl类是Process类的一个子类，start方法最后返回了一个ProcessImpl对象。而且ProcessImpl重写了waitFor方法。

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