来自Perl,我肯定错过了在源代码中创建多行字符串的“here-document”方法:

$string = <<"EOF"  # create a three-line string
text
text
text
EOF

在Java中,当我从头开始连接多行字符串时,我必须在每一行上使用繁琐的引号和加号。

有什么更好的选择吗?在属性文件中定义我的字符串?

编辑:有两个答案说StringBuilder.append()比加号更可取。谁能详细解释一下他们为什么这么想?在我看来,这一点也不可取。我正在寻找一种方法来解决多行字符串不是一级语言结构这一事实,这意味着我绝对不想用方法调用取代一级语言结构(字符串连接与加号)。

编辑:为了进一步澄清我的问题,我根本不关心性能。我关心的是可维护性和设计问题。


当前回答

如果你在属性文件中定义字符串,它看起来会更糟糕。IIRC,它看起来像:

string:text\u000atext\u000atext\u000a

一般来说,在源代码中不嵌入大字符串是一个合理的想法。您可能希望将它们作为资源加载,可能以XML或可读的文本格式加载。文本文件可以在运行时读取,也可以编译成Java源代码。如果你最终把它们放在源代码中,我建议把+放在前面,省略不必要的新行:

final String text = ""
    +"text "
    +"text "
    +"text"
;

如果你有新的行,你可能需要一些join或格式化方法:

final String text = join("\r\n"
    ,"text"
    ,"text"
    ,"text"
);

其他回答

Java 13及以上版本

Java中现在通过文本块支持多行字符串。在Java 13和14中,这个特性要求你在构建和运行项目时设置——enable-preview选项。在Java 15及以后版本中,由于文本块已成为标准特性,因此不再需要此选项。查看官方的文本块编程指南,了解更多细节。

现在,在Java 13之前,你是这样写查询的:

List<Tuple> posts = entityManager
.createNativeQuery(
    "SELECT *\n" +
    "FROM (\n" +
    "    SELECT *,\n" +
    "           dense_rank() OVER (\n" +
    "               ORDER BY \"p.created_on\", \"p.id\"\n" +
    "           ) rank\n" +
    "    FROM (\n" +
    "        SELECT p.id AS \"p.id\",\n" +
    "               p.created_on AS \"p.created_on\",\n" +
    "               p.title AS \"p.title\",\n" +
    "               pc.id as \"pc.id\",\n" +
    "               pc.created_on AS \"pc.created_on\",\n" +
    "               pc.review AS \"pc.review\",\n" +
    "               pc.post_id AS \"pc.post_id\"\n" +
    "        FROM post p\n" +
    "        LEFT JOIN post_comment pc ON p.id = pc.post_id\n" +
    "        WHERE p.title LIKE :titlePattern\n" +
    "        ORDER BY p.created_on\n" +
    "    ) p_pc\n" +
    ") p_pc_r\n" +
    "WHERE p_pc_r.rank <= :rank\n",
    Tuple.class)
.setParameter("titlePattern", "High-Performance Java Persistence %")
.setParameter("rank", 5)
.getResultList();

多亏了Java 13 Text Blocks,你可以像下面这样重写这个查询:

List<Tuple> posts = entityManager
.createNativeQuery("""
    SELECT *
    FROM (
        SELECT *,
               dense_rank() OVER (
                   ORDER BY "p.created_on", "p.id"
               ) rank
        FROM (
            SELECT p.id AS "p.id",
                   p.created_on AS "p.created_on",
                   p.title AS "p.title",
                   pc.id as "pc.id",
                   pc.created_on AS "pc.created_on",
                   pc.review AS "pc.review",
                   pc.post_id AS "pc.post_id"
            FROM post p
            LEFT JOIN post_comment pc ON p.id = pc.post_id
            WHERE p.title LIKE :titlePattern
            ORDER BY p.created_on
        ) p_pc
    ) p_pc_r
    WHERE p_pc_r.rank <= :rank
    """,
    Tuple.class)
.setParameter("titlePattern", "High-Performance Java Persistence %")
.setParameter("rank", 5)
.getResultList();

可读性强多了,对吧?

支持的想法

IntelliJ IDEA提供了将传统字符串连接块转换为新的多行字符串格式的支持:

Json, html, XML

多行String在编写JSON、HTML或XML时特别有用。

考虑下面的例子,使用String连接来构建一个JSON字符串文字:

entityManager.persist(
    new Book()
    .setId(1L)
    .setIsbn("978-9730228236")
    .setProperties(
        "{" +
        "   \"title\": \"High-Performance Java Persistence\"," +
        "   \"author\": \"Vlad Mihalcea\"," +
        "   \"publisher\": \"Amazon\"," +
        "   \"price\": 44.99," +
        "   \"reviews\": [" +
        "       {" +
        "           \"reviewer\": \"Cristiano\", " +
        "           \"review\": \"Excellent book to understand Java Persistence\", " +
        "           \"date\": \"2017-11-14\", " +
        "           \"rating\": 5" +
        "       }," +
        "       {" +
        "           \"reviewer\": \"T.W\", " +
        "           \"review\": \"The best JPA ORM book out there\", " +
        "           \"date\": \"2019-01-27\", " +
        "           \"rating\": 5" +
        "       }," +
        "       {" +
        "           \"reviewer\": \"Shaikh\", " +
        "           \"review\": \"The most informative book\", " +
        "           \"date\": \"2016-12-24\", " +
        "           \"rating\": 4" +
        "       }" +
        "   ]" +
        "}"
    )
);

由于转义字符和大量的双引号和加号,您几乎无法阅读JSON。

使用Java文本块,JSON对象可以这样写:

entityManager.persist(
    new Book()
    .setId(1L)
    .setIsbn("978-9730228236")
    .setProperties("""
        {
           "title": "High-Performance Java Persistence",
           "author": "Vlad Mihalcea",
           "publisher": "Amazon",
           "price": 44.99,
           "reviews": [
               {
                   "reviewer": "Cristiano",
                   "review": "Excellent book to understand Java Persistence",
                   "date": "2017-11-14",
                   "rating": 5
               },
               {
                   "reviewer": "T.W",
                   "review": "The best JPA ORM book out there",
                   "date": "2019-01-27",
                   "rating": 5
               },
               {
                   "reviewer": "Shaikh",
                   "review": "The most informative book",
                   "date": "2016-12-24",
                   "rating": 4
               }
           ]
        }
        """
    )
);

自从我在2004年使用c#以来,我一直希望在Java中有这个功能,现在我们终于有了它。

一个简单的选择是使用SciTE (http://www.scintilla.org/SciTEDownload.html)这样的编辑器编辑java代码,它允许您对文本进行WRAP,以便容易地查看和编辑长字符串。如果你需要转义字符,你只需输入它们。通过关闭“换行”选项,您可以检查字符串是否仍然是一个很长的单行字符串。当然,如果不是,编译器也会告诉你。

Eclipse或NetBeans是否支持编辑器中的文本包装,我不知道,因为它们有太多的选项。但如果没有,这将是一个很好的补充。

我所知道的唯一方法是用加号连接多行

我建议使用ThomasP所建议的实用程序,然后将其链接到构建过程中。仍然存在一个外部文件以包含文本,但在运行时不读取该文件。 工作流程如下:

构建一个“文本文件到java代码”实用程序并检查到版本控制 在每次构建时,针对资源文件运行实用程序以创建修改后的java源 Java源代码包含类似于 类TextBlock{…后面跟着一个从资源文件自动生成的静态字符串 用剩下的代码构建生成的java文件

一个小技巧。使用这个,我在动态创建的HTML页面中注入javascript

StringBuilder builder = new StringBuilder();

public String getString()
{
    return builder.toString();
}
private DropdownContent _(String a)
{
    builder.append(a);
    return this;
}

public String funct_showhide()
{
   return
    _("function slidedown_showHide(boxId)").
    _("{").
    _("if(!slidedown_direction[boxId])slidedown_direction[boxId] = 1;").
    _("if(!slideDownInitHeight[boxId])slideDownInitHeight[boxId] = 0;").
    _("if(slideDownInitHeight[boxId]==0)slidedown_direction[boxId]=slidedownSpeed; ").
    _("else slidedown_direction[boxId] = slidedownSpeed*-1;").
    _("slidedownContentBox = document.getElementById(boxId);").
    _("var subDivs = slidedownContentBox.getElementsByTagName('DIV');").
    _("for(var no=0;no<subDivs.length;no++){").
    _(" if(subDivs[no].className=='dhtmlgoodies_content')slidedownContent = subDivs[no];").
    _("}").
    _("contentHeight = slidedownContent.offsetHeight;").
    _("slidedownContentBox.style.visibility='visible';").
    _("slidedownActive = true;").
    _("slidedown_showHide_start(slidedownContentBox,slidedownContent);").
    _("}").getString();

}