how to find path to Java source code files from JUnit tests execution via Bazel

One way to do this is to use the data attribute which makes files available to tests (and other binaries) at runtime, and for Java there's the Runfiles library provided with Bazel for finding data files. E.g.:

BUILD:

java_test(
  name = "TextFileTest",
  srcs = ["javatests/txtfile/TextFileTest.java"],
  data = ["text.txt"],
  deps = [
      "@bazel_tools//tools/java/runfiles:runfiles",
      "@maven//:junit_junit",
  ],
)

WORKSPACE:

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")

RULES_JVM_EXTERNAL_TAG = "4.2"
RULES_JVM_EXTERNAL_SHA = "cd1a77b7b02e8e008439ca76fd34f5b07aecb8c752961f9640dea15e9e5ba1ca"

http_archive(
    name = "rules_jvm_external",
    strip_prefix = "rules_jvm_external-%s" % RULES_JVM_EXTERNAL_TAG,
    sha256 = RULES_JVM_EXTERNAL_SHA,
    url = "https://github.com/bazelbuild/rules_jvm_external/archive/%s.zip" % RULES_JVM_EXTERNAL_TAG,
)

load("@rules_jvm_external//:defs.bzl", "maven_install")

maven_install(
    artifacts = [
        "junit:junit:4.13.2",
    ],
    repositories = [
        "https://repo1.maven.org/maven2",
    ],
)

javatests/txtfile/TextFileTest.java:

package txtfile;

import com.google.devtools.build.runfiles.Runfiles;

import java.nio.file.Files;
import java.nio.file.Paths;
import java.nio.file.Path;

import org.junit.Assert;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.junit.runners.JUnit4;

@RunWith(JUnit4.class)
public class TextFileTest {

  @Test
  public void testTextFile() throws Exception {
    Runfiles runfiles = Runfiles.create();
    Path path = Paths.get(runfiles.rlocation("__main__/text.txt"));
    String line = Files.readAllLines(path).get(0);
    Assert.assertEquals(line, "this is text");
  }
}

text.txt:

this is text

$ bazel test TextFileTest
INFO: Build option --test_sharding_strategy has changed, discarding analysis cache.
INFO: Analyzed target //:TextFileTest (0 packages loaded, 547 targets configured).
INFO: Found 1 test target...
Target //:TextFileTest up-to-date:
  bazel-bin/TextFileTest.jar
  bazel-bin/TextFileTest
INFO: Elapsed time: 0.288s, Critical Path: 0.01s
INFO: 1 process: 1 internal.
INFO: Build completed successfully, 1 total action
//:TextFileTest                                                 (cached) PASSED in 0.3s

Executed 0 out of 1 test: 1 test passes.
INFO: Build completed successfully, 1 total action

Note that in runfiles.rlocation("__main__/text.txt") the __main__ is the workspace name, which by default is __main__. If there's a workspace(name = "workspace name") in the WORKSPACE file, then the workspace name would be set from that.