java: use single javac invocation per jar

Instead of invoking javac for every .java file, pass all of the sources
for a jar target to a single javac invocation. This massively improves
first compilation time and doesn't meaningfully affect incremental builds
(it can even be faster in some cases).

The old approach also had issues where files would not always get recompiled
even though they should, necessitating a clean rebuild in order to see changes
reflected in the build output.

Multiple invocations seem to only make sense if:
- issues with files not getting flagged for rebuild are investigated and fixed
- something like the javaserver buildtool from openjdk sources is used
instead of directly spawning javac processes
- the amount of java files per jar is so large that it is faster to compile
several files one by one than to compile all the files at once (batching may
still make sense to get a reasonable balance)

1 files changed, 21 insertions, 13 deletions

diff --git a/mesonbuild/backend/ninjabackend.py b/mesonbuild/backend/ninjabackend.py
index 4d56f7d..9d43c5e 100644
--- a/mesonbuild/backend/ninjabackend.py
+++ b/mesonbuild/backend/ninjabackend.py
@@ -1412,7 +1412,6 @@ class NinjaBackend(backends.Backend):
         outname_rel = os.path.join(self.get_target_dir(target), fname)
         src_list = target.get_sources()
         resources = target.get_java_resources()
-        class_list = []
         compiler = target.compilers['java']
         c = 'c'
         m = 'm'
@@ -1430,10 +1429,8 @@ class NinjaBackend(backends.Backend):
             if rel_src.endswith('.java'):
                 gen_src_list.append(raw_src)
 
-        compile_args = self.determine_single_java_compile_args(target, compiler)
-        for src in src_list + gen_src_list:
-            plain_class_path = self.generate_single_java_compile(src, target, compiler, compile_args)
-            class_list.append(plain_class_path)
+        compile_args = self.determine_java_compile_args(target, compiler)
+        class_list = self.generate_java_compile(src_list + gen_src_list, target, compiler, compile_args)
         class_dep_list = [os.path.join(self.get_target_private_dir(target), i) for i in class_list]
         manifest_path = os.path.join(self.get_target_private_dir(target), 'META-INF', 'MANIFEST.MF')
         manifest_fullpath = os.path.join(self.environment.get_build_dir(), manifest_path)
@@ -1530,7 +1527,8 @@ class NinjaBackend(backends.Backend):
         self.generate_generator_list_rules(target)
         self.create_target_source_introspection(target, compiler, commands, rel_srcs, generated_rel_srcs)
 
-    def determine_single_java_compile_args(self, target, compiler):
+    def determine_java_compile_args(self, target, compiler):
+        args = []
         args = self.generate_basic_compiler_args(target, compiler)
         args += target.get_java_args()
         args += compiler.get_output_args(self.get_target_private_dir(target))
@@ -1544,20 +1542,30 @@ class NinjaBackend(backends.Backend):
         args += ['-sourcepath', sourcepath]
         return args
 
-    def generate_single_java_compile(self, src, target, compiler, args):
+    def generate_java_compile(self, srcs, target, compiler, args):
         deps = [os.path.join(self.get_target_dir(l), l.get_filename()) for l in target.link_targets]
         generated_sources = self.get_target_generated_sources(target)
         for rel_src in generated_sources.keys():
             if rel_src.endswith('.java'):
                 deps.append(rel_src)
-        rel_src = src.rel_to_builddir(self.build_to_src)
-        plain_class_path = src.fname[:-4] + 'class'
-        rel_obj = os.path.join(self.get_target_private_dir(target), plain_class_path)
-        element = NinjaBuildElement(self.all_outputs, rel_obj, self.compiler_to_rule_name(compiler), rel_src)
+
+        rel_srcs = []
+        plain_class_paths = []
+        rel_objs = []
+        for src in srcs:
+            rel_src = src.rel_to_builddir(self.build_to_src)
+            rel_srcs.append(rel_src)
+
+            plain_class_path = src.fname[:-4] + 'class'
+            plain_class_paths.append(plain_class_path)
+            rel_obj = os.path.join(self.get_target_private_dir(target), plain_class_path)
+            rel_objs.append(rel_obj)
+        element = NinjaBuildElement(self.all_outputs, rel_objs, self.compiler_to_rule_name(compiler), rel_srcs)
         element.add_dep(deps)
         element.add_item('ARGS', args)
+        element.add_item('FOR_JAR', self.get_target_filename(target))
         self.add_build(element)
-        return plain_class_path
+        return plain_class_paths
 
     def generate_java_link(self):
         rule = 'java_LINKER'
@@ -2376,7 +2384,7 @@ class NinjaBackend(backends.Backend):
     def generate_java_compile_rule(self, compiler):
         rule = self.compiler_to_rule_name(compiler)
         command = compiler.get_exelist() + ['$ARGS', '$in']
-        description = 'Compiling Java object $in'
+        description = 'Compiling Java sources for $FOR_JAR'
         self.add_rule(NinjaRule(rule, command, [], description))
 
     def generate_cs_compile_rule(self, compiler: 'CsCompiler') -> None: