Panduan untuk API java.lang.ProcessBuilder

1. Gambaran keseluruhan

API Proses menyediakan cara yang kuat untuk melaksanakan perintah sistem operasi di Java. Walau bagaimanapun, ia mempunyai beberapa pilihan yang menjadikannya sukar untuk digunakan.

Dalam tutorial ini, kita akan melihat bagaimana Java meringankannya dengan ProcessBuilder API.

2. API ProcessBuilder

The ProcessBuilder kelas menyediakan kaedah untuk mencipta dan mengkonfigurasi proses sistem operasi. Setiap contoh ProcessBuilder membolehkan kami menguruskan koleksi atribut proses . Kita kemudian boleh memulakan Proses baru dengan atribut yang diberikan.

Berikut adalah beberapa senario biasa di mana kami dapat menggunakan API ini:

  • Cari versi Java semasa
  • Sediakan peta nilai kunci khusus untuk persekitaran kita
  • Tukar direktori kerja di mana perintah shell kami dijalankan
  • Alihkan aliran input dan output ke penggantian tersuai
  • Waris kedua-dua aliran proses JVM semasa
  • Jalankan perintah shell dari kod Java

Kami akan melihat contoh praktikal untuk masing-masing di bahagian kemudian.

Tetapi sebelum kita menyelidiki kod kerja, mari kita lihat jenis fungsi yang disediakan oleh API ini.

2.1. Ringkasan Kaedah

Pada bahagian ini, kita akan mengambil langkah mundur dan melihat secara ringkas kaedah yang paling penting dalam kelas ProcessBuilder . Ini akan membantu kita ketika kita menyelidiki beberapa contoh sebenar di kemudian hari:

  • ProcessBuilder(String... command)

    Untuk membuat pembangun proses baru dengan program dan argumen sistem operasi yang ditentukan, kita dapat menggunakan pembina yang mudah ini.

  • directory(File directory)

    Kita dapat mengatasi direktori kerja lalai dari proses semasa dengan memanggil kaedah direktori dan meneruskan objek File . Secara lalai, direktori kerja semasa ditetapkan ke nilai yang dikembalikan oleh sifat sistem user.dir .

  • environment()

    Sekiranya kita ingin mendapatkan pemboleh ubah persekitaran semasa, kita boleh memanggil kaedah persekitaran . Ia mengembalikan salinan persekitaran proses semasa menggunakan System.getenv () tetapi sebagai Peta .

  • inheritIO()

    Sekiranya kita ingin menentukan bahawa sumber dan destinasi untuk I / O standard subproses kita harus sama dengan proses Java semasa, kita dapat menggunakan kaedah pewarisan .

  • redirectInput(File file), redirectOutput(File file), redirectError(File file)

    Apabila kami ingin mengarahkan semula input, output dan tujuan ralat pembangun proses ke fail, kami mempunyai tiga kaedah pengalihan yang serupa ini.

  • start()

    Akhir sekali, untuk memulakan proses baru dengan apa yang telah kami konfigurasikan, kami hanya memanggil permulaan () .

Kita harus ambil perhatian bahawa kelas ini TIDAK disegerakkan . Sebagai contoh, jika kita mempunyai banyak utas mengakses instance ProcessBuilder secara serentak, maka penyegerakan mesti dikendalikan secara luaran.

3. Contohnya

Setelah kita memahami asas API ProcessBuilder , mari kita teliti beberapa contoh.

3.1. Menggunakan ProcessBuilder untuk Mencetak Versi Java

Dalam contoh pertama ini, kita akan menjalankan perintah java dengan satu argumen untuk mendapatkan versi .

Process process = new ProcessBuilder("java", "-version").start();

Pertama, kami membuat objek ProcessBuilder kami yang meneruskan nilai perintah dan argumen kepada pembina. Seterusnya, kita memulakan proses menggunakan kaedah start () untuk mendapatkan objek Proses .

Sekarang mari kita lihat bagaimana menangani output:

List results = readOutput(process.getInputStream()); assertThat("Results should not be empty", results, is(not(empty()))); assertThat("Results should contain java version: ", results, hasItem(containsString("java version"))); int exitCode = process.waitFor(); assertEquals("No errors should be detected", 0, exitCode);

Di sini kita membaca output proses dan mengesahkan kandungannya seperti yang kita harapkan. Pada langkah terakhir, kami menunggu proses selesai menggunakan proses.waitFor () .

Setelah proses selesai, nilai pengembalian memberitahu kita sama ada proses itu berjaya atau tidak .

Beberapa perkara penting yang perlu diingat:

  • Hujah mesti mengikut urutan yang betul
  • Lebih-lebih lagi, dalam contoh ini, direktori dan persekitaran kerja lalai digunakan
  • Kami sengaja tidak memanggil proses.waitFor () sehingga setelah kami membaca output kerana buffer output mungkin menghentikan prosesnya
  • Kami telah membuat andaian bahawa arahan java tersedia melalui pemboleh ubah PATH

3.2. Memulakan Proses Dengan Persekitaran yang Diubahsuai

Dalam contoh seterusnya, kita akan melihat bagaimana mengubah persekitaran kerja.

Tetapi sebelum kita melakukannya, mulailah dengan melihat jenis maklumat yang dapat kita temui di persekitaran lalai :

ProcessBuilder processBuilder = new ProcessBuilder(); Map environment = processBuilder.environment(); environment.forEach((key, value) -> System.out.println(key + value));

Ini hanya mencetak setiap entri pemboleh ubah yang disediakan secara lalai:

PATH/usr/bin:/bin:/usr/sbin:/sbin SHELL/bin/bash ...

Now we're going to add a new environment variable to our ProcessBuilder object and run a command to output its value:

environment.put("GREETING", "Hola Mundo"); processBuilder.command("/bin/bash", "-c", "echo $GREETING"); Process process = processBuilder.start();

Let’s decompose the steps to understand what we've done:

  • Add a variable called ‘GREETING' with a value of ‘Hola Mundo' to our environment which is a standard Map
  • This time, rather than using the constructor we set the command and arguments via the command(String… command) method directly.
  • We then start our process as per the previous example.

To complete the example, we verify the output contains our greeting:

List results = readOutput(process.getInputStream()); assertThat("Results should not be empty", results, is(not(empty()))); assertThat("Results should contain java version: ", results, hasItem(containsString("Hola Mundo")));

3.3. Starting a Process With a Modified Working Directory

Sometimes it can be useful to change the working directory. In our next example we're going to see how to do just that:

@Test public void givenProcessBuilder_whenModifyWorkingDir_thenSuccess() throws IOException, InterruptedException { ProcessBuilder processBuilder = new ProcessBuilder("/bin/sh", "-c", "ls"); processBuilder.directory(new File("src")); Process process = processBuilder.start(); List results = readOutput(process.getInputStream()); assertThat("Results should not be empty", results, is(not(empty()))); assertThat("Results should contain directory listing: ", results, contains("main", "test")); int exitCode = process.waitFor(); assertEquals("No errors should be detected", 0, exitCode); }

In the above example, we set the working directory to the project's src dir using the convenience method directory(File directory). We then run a simple directory listing command and check that the output contains the subdirectories main and test.

3.4. Redirecting Standard Input and Output

In the real world, we will probably want to capture the results of our running processes inside a log file for further analysis. Luckily the ProcessBuilder API has built-in support for exactly this as we will see in this example.

By default, our process reads input from a pipe. We can access this pipe via the output stream returned by Process.getOutputStream().

However, as we'll see shortly, the standard output may be redirected to another source such as a file using the method redirectOutput. In this case, getOutputStream() will return a ProcessBuilder.NullOutputStream.

Let's return to our original example to print out the version of Java. But this time let's redirect the output to a log file instead of the standard output pipe:

ProcessBuilder processBuilder = new ProcessBuilder("java", "-version"); processBuilder.redirectErrorStream(true); File log = folder.newFile("java-version.log"); processBuilder.redirectOutput(log); Process process = processBuilder.start();

In the above example, we create a new temporary file called log and tell our ProcessBuilder to redirect output to this file destination.

In this last snippet, we simply check that getInputStream() is indeed null and that the contents of our file are as expected:

assertEquals("If redirected, should be -1 ", -1, process.getInputStream().read()); List lines = Files.lines(log.toPath()).collect(Collectors.toList()); assertThat("Results should contain java version: ", lines, hasItem(containsString("java version")));

Now let's take a look at a slight variation on this example. For example when we wish to append to a log file rather than create a new one each time:

File log = tempFolder.newFile("java-version-append.log"); processBuilder.redirectErrorStream(true); processBuilder.redirectOutput(Redirect.appendTo(log));

It's also important to mention the call to redirectErrorStream(true). In case of any errors, the error output will be merged into the normal process output file.

We can, of course, specify individual files for the standard output and the standard error output:

File outputLog = tempFolder.newFile("standard-output.log"); File errorLog = tempFolder.newFile("error.log"); processBuilder.redirectOutput(Redirect.appendTo(outputLog)); processBuilder.redirectError(Redirect.appendTo(errorLog));

3.5. Inheriting the I/O of the Current Process

In this penultimate example, we'll see the inheritIO() method in action. We can use this method when we want to redirect the sub-process I/O to the standard I/O of the current process:

@Test public void givenProcessBuilder_whenInheritIO_thenSuccess() throws IOException, InterruptedException { ProcessBuilder processBuilder = new ProcessBuilder("/bin/sh", "-c", "echo hello"); processBuilder.inheritIO(); Process process = processBuilder.start(); int exitCode = process.waitFor(); assertEquals("No errors should be detected", 0, exitCode); }

In the above example, by using the inheritIO() method we see the output of a simple command in the console in our IDE.

In the next section, we're going to take a look at what additions were made to the ProcessBuilder API in Java 9.

4. Java 9 Additions

Java 9 introduced the concept of pipelines to the ProcessBuilder API:

public static List startPipeline​(List builders) 

Using the startPipeline method we can pass a list of ProcessBuilder objects. This static method will then start a Process for each ProcessBuilder. Thus, creating a pipeline of processes which are linked by their standard output and standard input streams.

For example, if we want to run something like this:

find . -name *.java -type f | wc -l

What we'd do is create a process builder for each isolated command and compose them into a pipeline:

@Test public void givenProcessBuilder_whenStartingPipeline_thenSuccess() throws IOException, InterruptedException { List builders = Arrays.asList( new ProcessBuilder("find", "src", "-name", "*.java", "-type", "f"), new ProcessBuilder("wc", "-l")); List processes = ProcessBuilder.startPipeline(builders); Process last = processes.get(processes.size() - 1); List output = readOutput(last.getInputStream()); assertThat("Results should not be empty", output, is(not(empty()))); }

In this example, we're searching for all the java files inside the src directory and piping the results into another process to count them.

To learn about other improvements made to the Process API in Java 9, check out our great article on Java 9 Process API Improvements.

5. Conclusion

To summarize, in this tutorial, we’ve explored the java.lang.ProcessBuilder API in detail.

First, we started by explaining what can be done with the API and summarized the most important methods.

Next, we took a look at a number of practical examples. Finally, we looked at what new additions were introduced to the API in Java 9.

Seperti biasa, kod sumber penuh artikel terdapat di GitHub.