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
使用Properties.loadFromXML (InputStream)。不需要外部库。
比起混乱的代码(因为可维护性和设计是您所关心的)，最好不要使用长字符串。 从读取xml属性开始:
InputStream fileIS = YourClass.class.getResourceAsStream("MultiLine.xml");
Properties prop = new Properies();
prop.loadFromXML(fileIS);
然后，您可以使用您的多行字符串在一个更可维护的方式…
static final String UNIQUE_MEANINGFUL_KEY = "Super Duper UNIQUE Key";
prop.getProperty(UNIQUE_MEANINGFUL_KEY) // "\n MEGA\n LONG\n..."
MultiLine.xml`位于同一个文件夹YourClass:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE properties SYSTEM "http://java.sun.com/dtd/properties.dtd">
<properties>
<entry key="Super Duper UNIQUE Key">
MEGA
LONG
MULTILINE
</entry>
</properties>
注:你可以使用<![CDATA[”…"]]>为类似xml的字符串。
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。
我有时使用一个并行groovy类来充当一个字符串包
这里的java类
public class Test {
public static void main(String[] args) {
System.out.println(TestStrings.json1);
// consume .. parse json
}
}
以及TestStrings.groovy中令人垂涎的多行字符串
class TestStrings {
public static String json1 = """
{
"name": "Fakeer's Json",
"age":100,
"messages":["msg 1","msg 2","msg 3"]
}""";
}
当然，这只适用于静态字符串。如果我必须在文本中插入变量，我会将整个文件更改为groovy。只要保持强类型实践，它就可以实现。
你可以在一个单独的方法中连接你的追加:
public static String multilineString(String... lines){
StringBuilder sb = new StringBuilder();
for(String s : lines){
sb.append(s);
sb.append ('\n');
}
return sb.toString();
}
无论哪种方式，都更喜欢StringBuilder而不是加号符号。
遗憾的是，Java没有多行字符串字面量。您要么必须连接字符串字面量(使用+或StringBuilder是最常见的两种方法)，要么从单独的文件中读取字符串。
对于大的多行字符串文字，我倾向于使用一个单独的文件，并使用getResourceAsStream() (Class类的一个方法)读取它。这使得查找文件变得很容易，因为您不必担心当前目录与代码安装的位置。它还使打包更容易，因为您实际上可以将文件存储在jar文件中。
假设你在一个名为Foo的类中。就像这样做:
Reader r = new InputStreamReader(Foo.class.getResourceAsStream("filename"), "UTF-8");
String s = Utils.readAll(r);
另一个烦恼是Java没有标准的“将这个Reader中的所有文本读入字符串”方法。写起来很简单:
public static String readAll(Reader input) {
StringBuilder sb = new StringBuilder();
char[] buffer = new char[4096];
int charsRead;
while ((charsRead = input.read(buffer)) >= 0) {
sb.append(buffer, 0, charsRead);
}
input.close();
return sb.toString();
}
import org.apache.commons.lang3.StringUtils;
String multiline = StringUtils.join(new String[] {
"It was the best of times, it was the worst of times ",
"it was the age of wisdom, it was the age of foolishness",
"it was the epoch of belief, it was the epoch of incredulity",
"it was the season of Light, it was the season of Darkness",
"it was the spring of hope, it was the winter of despair",
"we had everything before us, we had nothing before us",
}, "\n");
