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:
$string = <<"EOF" # create a three-line string
text
text
text
EOF
在Java中，当我从头开始连接多行字符串时，我必须在每一行上使用繁琐的引号和加号。
有什么更好的选择吗?在属性文件中定义我的字符串?
编辑:有两个答案说StringBuilder.append()比加号更可取。谁能详细解释一下他们为什么这么想?在我看来，这一点也不可取。我正在寻找一种方法来解决多行字符串不是一级语言结构这一事实，这意味着我绝对不想用方法调用取代一级语言结构(字符串连接与加号)。
编辑:为了进一步澄清我的问题，我根本不关心性能。我关心的是可维护性和设计问题。
当前回答
另一种方法是将长字符串存储在外部文件中，然后将文件读入字符串。
其他回答
你可以使用scala-code，它与java兼容，并且允许用""" "括起来的多行字符串:
package foobar
object SWrap {
def bar = """John said: "This is
a test
a bloody test,
my dear." and closed the door."""
}
(注意字符串内的引号)和来自java:
String s2 = foobar.SWrap.bar ();
这样是否更舒服?
另一种方法，如果你经常处理长文本，应该放在你的源代码中，可能是一个脚本，它从外部文件中获取文本，并将其包装为一个多行java- string，像这样:
sed '1s/^/String s = \"/;2,$s/^/\t+ "/;2,$s/$/"/' file > file.java
这样你就可以很容易地复制粘贴到你的源代码中。
注意:这个答案适用于Java 14及以上版本。
Java 15引入了文本块(多行文字)。详情请看这个答案。
听起来像是要做一个多行文字，这在Java中是不存在的。
你最好的选择是用+ d组合的字符串。人们提到的一些其他选项(StringBuilder, String。format, String.join)只有当你从一个字符串数组开始时才更可取。
考虑一下:
String s = "It was the best of times, it was the worst of times,\n"
+ "it was the age of wisdom, it was the age of foolishness,\n"
+ "it was the epoch of belief, it was the epoch of incredulity,\n"
+ "it was the season of Light, it was the season of Darkness,\n"
+ "it was the spring of hope, it was the winter of despair,\n"
+ "we had everything before us, we had nothing before us";
与StringBuilder:
String s = new StringBuilder()
.append("It was the best of times, it was the worst of times,\n")
.append("it was the age of wisdom, it was the age of foolishness,\n")
.append("it was the epoch of belief, it was the epoch of incredulity,\n")
.append("it was the season of Light, it was the season of Darkness,\n")
.append("it was the spring of hope, it was the winter of despair,\n")
.append("we had everything before us, we had nothing before us")
.toString();
与String.format ():
String s = String.format("%s\n%s\n%s\n%s\n%s\n%s"
, "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"
);
相对于Java8 String.join():
String s = String.join("\n"
, "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"
);
如果您想在特定的系统中使用换行符，您要么需要使用system . lineseparator()，要么可以在String.format中使用%n。
另一种选择是将资源放在文本文件中，只读取该文件的内容。对于非常大的字符串，这将是可取的，以避免不必要地膨胀您的类文件。
这是你不应该在不考虑它在做什么的情况下使用的东西。但对于一次性脚本，我已经成功地使用了这个方法:
例子:
System.out.println(S(/*
This is a CRAZY " ' ' " multiline string with all sorts of strange
characters!
*/));
代码:
// From: http://blog.efftinge.de/2008/10/multi-line-string-literals-in-java.html
// Takes a comment (/**/) and turns everything inside the comment to a string that is returned from S()
public static String S() {
StackTraceElement element = new RuntimeException().getStackTrace()[1];
String name = element.getClassName().replace('.', '/') + ".java";
StringBuilder sb = new StringBuilder();
String line = null;
InputStream in = classLoader.getResourceAsStream(name);
String s = convertStreamToString(in, element.getLineNumber());
return s.substring(s.indexOf("/*")+2, s.indexOf("*/"));
}
// From http://www.kodejava.org/examples/266.html
private static String convertStreamToString(InputStream is, int lineNum) {
/*
* To convert the InputStream to String we use the BufferedReader.readLine()
* method. We iterate until the BufferedReader return null which means
* there's no more data to read. Each line will appended to a StringBuilder
* and returned as String.
*/
BufferedReader reader = new BufferedReader(new InputStreamReader(is));
StringBuilder sb = new StringBuilder();
String line = null; int i = 1;
try {
while ((line = reader.readLine()) != null) {
if (i++ >= lineNum) {
sb.append(line + "\n");
}
}
} catch (IOException e) {
e.printStackTrace();
} finally {
try {
is.close();
} catch (IOException e) {
e.printStackTrace();
}
}
return sb.toString();
}
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。
我所知道的唯一方法是用加号连接多行
